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A spoon scooping fennel and onion gratin out of a cast iron skillet.

Fennel and Onion Gratin

  • Author: Nicole Gaffney (
  • 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


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.


  • 4 tablespoons butter, divided, plus more for greasing the dish
  • 1 large or 2 smaller fennel bulbs, sliced (cored if very large and fibrous)
  • 2 large sweet onions, peeled, halved and sliced
  • 1 teaspoon salt, plus more to taste
  • freshly cracked black pepper to taste
  • 1 teaspoon fresh thyme leaves
  • 3/4 cup heavy cream
  • 1 cup finely grated pecorino Romano cheese, plus more for topping
  • 1 cup panko breadcrumbs


  1. Preheat the oven to 425 degrees.
  2. Melt two tablespoons butter in a large saucepan over medium heat. Add the fennel, season with salt, stir, then cover and let cook for about 10 minutes. Next, add the onions, stir, cover, and cook for another 15 minutes or until tender.
  3. while that’s cooking, melt the remaining two tablespoons butter in a medium saute pan. Add in the panko breadcrumbs and stir around until they’re golden and toasted. Set aside.
  4. When the onions and fennel are tender, pour in the heavy cream, season with salt, pepper and fresh thyme, then cook uncovered for a few minutes until the cream has reduced. Sprinkle in the pecorino Romano cheese, mix it all together and remove from the heat.
  5. Butter the inside of a small-medium casserole dish or cast iron skillet, then pour in the onions and fennel. Top with more cheese and the toasted breadcrumbs, then pop into the oven for about 20 minutes until its bubbling and golden brown on top.


  • *If making ahead, assemble completely, cover and store in the refrigerator until ready to cook – up to 3 days ahead. Lower the oven temp to 400 degrees and cook for an additional 10 minutes (about 30 total), until its bubbling around the sides.

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