• Skip to primary navigation
  • Skip to main content
  • Skip to primary sidebar
  • Skip to footer
Coley Cooks
  • About
  • Recipes
    • Appetizers
    • Breakfast
    • Casseroles
    • Chicken + Poultry
    • Cookies + Bars
    • Drinks
    • Frozen Treats
    • Pasta + Risotto
    • Pies + Tarts
    • Meat
    • Salads
    • Seafood
    • Snacks
    • Soups + Stews
    • Starchy Sides
    • Vegetable Sides
    • Vegetarian Main
  • Privacy Policy
  • Subscribe
menu icon
go to homepage
subscribe
search icon
Homepage link
  • About
  • Recipes
  • Privacy Policy
×

Home » Recipes » Casseroles

Fennel + Onion Gratin

Published: Nov 15, 2021 · Modified: Nov 15, 2021 by Coley · This post may contain affiliate links.

Jump to Recipe

This crispy, cheesy fennel and onion gratin is perfect for the holidays and chilly winter months. The onions and fennel get cooked down in butter to become super sweet, tender and mellow. They get mixed with sharp Pecorino Romano cheese, fresh thyme and a splash of cream, then get baked with an ultra crispy breadcrumb topping. It's an easy, make-ahead side that's lovely with steak, chicken, turkey, pork or fish. 

spoon full of fennel onion gratin in a cast iron skillet with fresh thyme

Why this recipe works

  • Inspired by the creamed peal onions that often show up on the Thanksgiving table, this is a more modern take that tastes sooo much better.
  • Onions and fennel get cooked down to become super sweet, tender and mellow. Even self proclaimed fennel haters will fall in love!
  • Pecorino Romano cheese adds a sharp, salty flavor that pairs perfectly with the sweet fennel and onions.
  • Toasting Panko breadcrumbs in butter before baking ensures a perfectly light and crisp topping with no soggy bits.
  • Can be made entirely in advance for easy entertaining or bringing to a potluck.
ingredients for fennel onion gratin on dark background

Essential Ingredients

  • Onions - I love making this with sweet onions such as Vidalia, but regular yellow onions will still work great.
  • Pecorino Romano Cheese - Locatelli brand is the gold standard - use it if you can find it. You can substitute parmesan, however it will have a more mild flavor.
  • Panko Breadcrumbs - These light, airy Japanese breadcrumbs make the best topping. Substitute homemade bread crumbs made from torn stale bread if you can't find them.

Step by Step Instructions

  • fennel being sliced on a mandolin on a wood cutting board
  • adding fennel to a pot with melted butter

1. Use a mandolin or sharp knife to thinly slice the onions and fennel (use only the white bulb of the fennel and avoid the green stems). If the fennel is very large, remove the core with a knife before slicing, as it can be tough.

2. Melt 2 tablespoons of butter in a large saucepan over medium heat, then add the fennel and give it a stir. Cover and let cook for about 10 minutes, stirring occasionally.

  • sautéing onions and fennel in butter in a pot with a wooden spoon
  • adding Panko to a cast iron skillet with melted butter

3. Add the onions, stir, then cover and let cook until tender, stirring occasionally, about 15 minutes.

4. While the fennel and onions cook, melt the remaining two tablespoons of butter in a medium saute pan. Add the Panko breadcrumbs and stir.

  • stirring toasted panko in a cast iron skillet with rubber spatula
  • pot of cooked fennel and onions with salt, pepper, heavy cream and thyme

5. Continue stirring the Panko until golden and toasted, about 3 minutes. Set aside to cool.

6. Once the fennel and onions are tender, add the heavy cream and fresh thyme, then season with salt and pepper to taste. Cook uncovered until the cream reduces by about ⅓, about 5 minutes.

  • stirring pecorino Romano cheese into pot with cooked fennel and onion
  • sprinting toasted panko over cast iron skillet with fennel and onions

7. Remove the pan from the heat and stir in the Pecorino Romano cheese.

8. Butter the inside of an 8-9 inch cast iron skillet or casserole dish, then pour in the mixture and smooth out the top. Sprinkle with more Pecorino Romano cheese, then the toasted breadcrumbs. If making the recipe ahead, let it cool, wrap tightly and refrigerate for up to 3 days before baking.

9. Bake in a preheated 425 degrees F oven for about 20 minutes or until it's bubbling around the edges. If baking from the refrigerator, lower the oven temp to 375 and bake for 30 minutes total.

close up of baked fennel onion gratin

Tips for Success

  • Use a mandolin slicer for best results. This will result in uniformly thick slices of onion and fennel which will have a better finished texture. Nervous about using one? A cut proof glove is your best friend.
  • If the top is browning too quickly while it bakes, tent the pan with foil to slow the process.
  • This recipe can be made (before baking) up to 3 days ahead. Wrap tightly and store in the refrigerator, then bake at 375 degrees F for about 30 minutes. I do not recommend freezing.

Faq

Can anything be substituted for the fennel? I hate the taste of licorice.

I promise there is virtually no detectable licorice flavor in the final dish. Trust me, I also hate licorice. However, if you can't find fennel or have an allergy, replace it with two more onions and the dish will still be great.

Can I substitute half and half or coconut milk for the heavy cream?

No, it simply won't work.

Can I use any other cheeses?

I love the sharp, salty bite of the Pecorino Romano up against the sweet onions and fennel, but feel free to have fun and experiment with different cheeses. I recommend sticking to hard cheeses, the sharper the better. You may need to add more salt depending on the cheese you choose.

Can this be made ahead and frozen?

No. Because of the cream, this dish will separate and get a curdled texture if frozen. It can be made ahead and stored in the refrigerator for up to 3 days before baking.

fennel onion gratin in a cast iron skillet with spoon and scoop taken out

Related Recipes

  • Cauliflower Gratin
  • Classic Potato Gratin
  • Kale Gratin
  • Celery Root + Parsnip Gratin

Did you make this recipe and LOVE it? Please leave a star ⭐️ rating and/or comment to help other readers. I absolutely love hearing from you and do my best to answer all questions and comments. I love seeing when you make my recipes, so please tag me @ColeyCooks on Instagram and I will repost!

Print
clock clock iconcutlery cutlery iconflag flag iconfolder folder iconinstagram instagram iconpinterest pinterest iconfacebook facebook iconprint print iconsquares squares iconheart heart iconheart solid heart solid icon
fennel onion gratin in a cast iron skillet with spoon and scoop taken out

Fennel and Onion Gratin

★★★★★ 5 from 1 reviews
  • Author: Nicole Gaffney (ColeyCooks.com)
  • Prep Time: 10 minutes
  • Cook Time: 50 minutes
  • Total Time: 1 hour
  • Yield: 6-8 servings 1x
  • Category: side dish
  • Method: oven
  • Cuisine: american
  • Diet: Vegetarian
Print Recipe
Pin Recipe

Description

This simple recipe for fennel and onion gratin is perfect for the holidays. The onions and fennel cook down to be super sweet with an ultra crispy top.


Ingredients

Scale
  • 4 tablespoons butter, divided, plus more for greasing the dish
  • 1 large (or 2 smaller fennel bulbs), thinly sliced (if very large, remove the core)*
  • 2 medium-large sweet onions, peeled, halved and thinly sliced
  • 1 teaspoon salt, plus more to taste
  • freshly cracked black pepper to taste
  • 1 teaspoon fresh thyme leaves (sub ¼ teaspoon dried)
  • ¾ cup heavy cream
  • 1 ¼ cups finely grated pecorino Romano cheese
  • 1 cup panko breadcrumbs

Instructions

  1. Preheat the oven to 425 degrees F.
  2. Melt two tablespoons of butter in a large saucepan over medium heat. Add the fennel, season with salt, stir, then cover and let cook for about 10 minutes.
  3. Add the onions, stir, cover, and cook, stirring occasionally, until tender, about 15 minutes.
  4. while the fennel and onion cook, melt the remaining two tablespoons of butter in a medium saute pan.
  5. Add in the panko breadcrumbs and stir around until they're golden and toasted. Set aside to cool.
  6. When the onions and fennel are tender, add the heavy cream and thyme, then season with salt and pepper.
  7. Cook uncovered for a few minutes until the cream has reduced by about ⅓, then remove from the heat and stir in 1 cup of Pecorino Romano cheese.
  8. Butter the inside of an 8-9 inch casserole dish or cast iron skillet, then add the fennel mixture and smooth out the top.
  9. Sprinkle with remaining Pecorino Romano cheese and then the toasted breadcrumbs.
  10. Bake for about 20 minutes or until its bubbling and golden brown on top.

Notes

  • *For best results, use a mandolin slicer to cut the fennel and onions
  • If making ahead, assemble completely, but do not bake. Cover and store in the refrigerator up to 3 days ahead. Bake at 375 degrees F for about 30 minutes until its bubbling around the sides. If the top starts to get too brown, tent it with foil.

Keywords: fennel, onion, gratin, cheesy, fennel, easy side dish

Did you make this recipe?

Tag @coleycooks on Instagram and hashtag it #coleycooks

« Rosemary Maple Roasted Pecans
Foolproof Flaky Pie Dough »

STAY IN THE LOOP! Sign up for my newsletter to get recipes and tips delivered straight to your inbox.

Reader Interactions

Comments

  1. Danielle

    November 13, 2015 at 4:22 pm

    I'm a fennel fanatic. The rest of my family, not so much. They're all licorice-haters, too, so I get all the black jelly beans. But I think they'll go for this gratin since it's milder when cooked, plus butter and cheese. I've got all this stuff so I'll give it a test-run tonight. yumyum!

    Reply
    • Danielle

      November 20, 2015 at 3:27 pm

      We LOVED this! Pecorino is a good match for it, and the toasty topping was ridiculous.

      Reply
      • Coley

        November 23, 2015 at 6:45 pm

        Oh, yes! I'm so glad you guys loved it. Makes me a very happy lady! 🙂

        Reply
  2. London

    November 14, 2015 at 10:29 pm

    Hi Coley! Your nickname is actually the same as my husband's (his name is Cohl!) I just wanted to let you know that I think your videos are awesome. You've got a fun and sweet personality that definitely comes through on them. So happy I stumbled across your blog 🙂

    Reply
    • Coley

      November 16, 2015 at 8:58 pm

      Thank you so much! I'm happy to have ya here 🙂

      Reply
  3. Coley

    November 09, 2021 at 3:39 pm

    Such a hit at Thanksgiving! Not licoricey at all

    ★★★★★

    Reply

Did you make this recipe? Leave a rating and review! Cancel reply

Your email address will not be published. Required fields are marked *

Recipe rating ★☆ ★☆ ★☆ ★☆ ★☆

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Primary Sidebar

  • Bloglovin
  • Facebook
  • Instagram
  • Pinterest
  • Twitter
  • YouTube

Hey! I'm Nicole Gaffney, but my friends all call me Coley. You should, too! Or call me Nicole. Whatever. Just don't call me late for dinner.

About Nicole Gaffney

Subscribe to my Newsletter for all the latest recipes!

Looking for something in particular?

Soups

bowl of bean soup with a spoon sticking out

Easy + Inexpensive 5 Ingredient Bean Soup

close up of vegetable minestrone soup with parsley pistou herb drizzle

Vegetable Minestrone with Parsley Pistou

A bowl of lobster bisque with a spoon

Lobster Bisque

Chicken poblano white bean chili in red bowls. Tortilla strips, cilantro, and sour cream make wonderful toppings.

Chicken Poblano White Bean Chili

Hands cupping a bowl of chicken white bean soup.

30 Minute Pesto Chicken, Kale + White Bean Soup

Top view of three bowls of spicy carrot soup garnished with yogurt swirls.

20 Minute Spicy Carrot Soup with Yogurt + Sesame

Salads

overhead shot of grilled halloumi and peaches with arugula, jalapeños and honey lemon vinaigrette

Grilled Halloumi + Peaches with Arugula, Jalapeños and a Honey Lemon Vinaigrette

A plate of spinach salad with a fork and a hand

Warm Spinach Salad with Pancetta + Goat Cheese

Thai flavored papaya salad piled up on a blue plate with a fork.

Thai Green Papaya Salad

Oval serving platter with a fork next to green bean tuna salad with lemon wedges.

Italian Tuna + Green Bean Salad

Top view of a serving bowl with raw corn and nectarine salad.

Raw Corn + Nectarine Salad

Vertical shot of a tan plate with a fork and serving of lyonnaise salad.

Lyonnaise Salad

Breakfast

Perfect Muffin Tin Popovers (Video!)

Perfect Muffin Tin Popovers

Six square pieces of smoked salmon puff pastry tart on a serving platter.

Easy Smoked Salmon Puff Pastry Tart

A sunny side up egg over a pile of sweet potato bacon hash.

Sweet Potato + Bacon Hash with Crispy Fried Eggs (Video!)

Close up side view of a glass of cold brew coffee swirled with milk and ice cubes.

Cold Brew Coffee

close up of scrambled eggs on a plate with a fork

Soft Scrambled Eggs with Ricotta

Waffles with Maple Brown Butter

Waffles with Maple Brown Butter

Vegetable Sides

Top view of a square bowl of Thai cucumber salad with a spoon.

Thai Inspired Cucumber Salad

spoon full of fennel onion gratin in a cast iron skillet with fresh thyme

Fennel + Onion Gratin

Shaved Summer Squash Salad with Thai Basil + Peanuts (Video!)

Thai Shaved Zucchini Salad

Summer Corn + Green Beans with Herb Butter (Video!)

Summer Corn + Green Beans with Herb Butter

Serving bowl with wilted spinach topped with butter and Parmesan.

Spinach with Butter + Parmesan

Roasted Cauliflower with Breadcrumbs, Capers, Almonds + Raisins

Roasted Cauliflower with Breadcrumbs, Capers, Almonds + Raisins

Winter Recipes

Top view of a flourless chocolate cake with a cracked top and cardamom pods

Turkish Coffee Flourless Chocolate Cake

Top view close up of chocolate chunk cookies on a wood table.

Crispy Chewy Chocolate Chip Cookies

a plate of cookies with a broken cookie in the foreground and ingredients in the background

Mom's Chewy Oatmeal Chocolate Chip Cookies

bowl of bean soup with a spoon sticking out

Easy + Inexpensive 5 Ingredient Bean Soup

a loaf of cinnamon raisin walnut bread sliced

Cinnamon Raisin Walnut No-Knead Bread

close up of vegetable minestrone soup with parsley pistou herb drizzle

Vegetable Minestrone with Parsley Pistou

The Art of the Smoothie Bowl Cook Book is Out NOW!! Click the photo above to order.

Footer

Nicole Gaffney, aka Coley, is a professionally trained chef, recipe developer, tv host and food writer. Learn More about Coley…

Stay Connected!

  • Bloglovin
  • Facebook
  • Instagram
  • Pinterest
  • Twitter
  • YouTube

Didn't find what you were looking for?

Footer

↑ back to top

About

  • Privacy Policy

Newsletter

  • Sign Up! for emails and updates

Contact

  • Contact

As an Amazon Associate I earn from qualifying purchases.

Copyright © 2022 Brunch Pro on the Brunch Pro Theme