Fennel + Onion Gratin
This crispy, cheesy fennel and onion gratin is perfect for the holidays or chilly winter months. Sweet, tender fennel and onions are cooked in butter, mixed with Pecorino Romano, thyme, and cream, then baked under a golden breadcrumb topping. It's an easy make-ahead side, it pairs beautifully with steak, chicken, turkey, pork, or fish.
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For more vegetable sides, try my goat cheese stuffed tomatoes, grilled marinated eggplant, or Italian beans and greens.
Why This Recipe Works
- 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.
- Can be made entirely in advance for easy entertaining or bringing to a potluck.
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.
*Find the full ingredient list in the recipe card below!
Helpful Equipment
How to make Fennel + Onion Gratin
- 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.
- 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.
- Add the onions, stir, then cover and let cook until tender, stirring occasionally, about 15 minutes.
- While the fennel and onions cook, melt the remaining two tablespoons of butter in a medium sauté pan. Add the Panko breadcrumbs and stir.
- Continue stirring the Panko until golden and toasted, about 3 minutes. Set aside to cool.
- 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.
- Remove the pan from the heat and stir in the Pecorino Romano cheese.
- 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.
- Bake in a preheated 425°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°F and bake for 30 minutes total.
Tips for Success
- 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°F for about 30 minutes. I do not recommend freezing.
FAQs
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.
No, it simply won't work.
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.
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.
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Fennel and Onion Gratin
Ingredients
- 4 tablespoons butter divided, plus more for greasing the dish
- 1 large fennel bulbs or 2 small, thinly sliced (if very large, remove the core)*
- 2 sweet onions medium-large, peeled, halved and thinly sliced
- 1 teaspoon salt plus more to taste
- freshly cracked black pepper to taste
- 1 teaspoon fresh thyme leaves or ¼ teaspoon dried
- ¾ cup heavy cream
- 1¼ cups Pecorino Romano cheese finely grated
- 1 cup panko breadcrumbs
Instructions
- Preheat the oven to 425℉.
- 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.
- Add the onions, stir, cover, and cook, stirring occasionally, until tender, about 15 minutes.
- while the fennel and onion cook, melt the remaining two tablespoons of butter in a medium saute pan.
- Add in the panko breadcrumbs and stir around until they're golden and toasted. Set aside to cool.
- When the onions and fennel are tender, add the heavy cream and thyme, then season with salt and pepper.
- 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.
- Butter the inside of an 8-9 inch casserole dish or cast iron skillet, then add the fennel mixture and smooth out the top.
- Sprinkle with remaining Pecorino Romano cheese and then the toasted breadcrumbs.
- 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.
Can you bake in an au gratin dish? Or must it be cast iron?
Yes! It works in either one.
Good recipe. Especially using the Romano cheese. The sharp flavor enhances the sweetness developed by sautéing the fennel and onions. I didn’t brown the Italian seasoned panko bread crumbs I used, but they stayed crisp. I did spray with butter flavored Pam out of an issue of time management. My husband who doesn’t like fennel raw, loved this.
SO glad you enjoyed the recipe! Thanks for leaving such a detailed review. 🙂
Such a hit at Thanksgiving! Not licoricey at all
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 🙂
Thank you so much! I'm happy to have ya here 🙂
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!
We LOVED this! Pecorino is a good match for it, and the toasty topping was ridiculous.
Oh, yes! I'm so glad you guys loved it. Makes me a very happy lady! 🙂