Golden, buttery croissants meet melty Gruyère and sweet, jammy onions in this savory make-ahead brunch bake.
If you’re looking for a cozy, crowd-pleasing breakfast that feels just a little bit fancy, this is the one. Think of it as a French onion soup–inspired casserole—layers of torn croissants soaked in a rich, creamy custard, studded with deeply caramelized onions and topped with a melty blanket of Gruyère. It’s buttery, savory, and absolutely perfect for slow mornings, special brunches, or even breakfast-for-dinner kind of nights.
Why You’ll Love This Recipe
- Effortless but elegant – Store-bought croissants give it richness without any fuss.
- Make-ahead friendly – Soaks overnight like a dream. Just top with cheese and prosciutto, then bake and serve.
- Flavor-packed – Caramelized onions bring depth and sweetness, thyme brings herbal notes and Gruyère brings the cheesy, melty magic.
- Versatile – Serve it with a crisp arugula salad, a little bubbly, or a cozy coffee moment for a nice brunch
- Perfect for holidays - Make it the night before and bake in the morning. The longer it sets, the better it gets. And it tastes great at room temperature.
Ingredients You’ll Need
For the caramelized onions:
- Olive oil
- Yellow onions
- Kosher salt
- Balsamic or sherry vinegar (optional, but adds a beautiful finish)
For the casserole:
- Croissants (slightly stale ones are perfect!)
- Gruyère cheese + Prosciutto for topping
- Eggs, milk, and cream, Dijon mustard, dried thyme, garlic powder, salt, and pepper
📌 Full recipe card is at the bottom of this post.
Step-by-Step Instructions
1. Caramelize the Onions
Take your time with this step—it’s where all the flavor builds. Slowly cook thinly sliced onions in olive oil or butter with a pinch of salt until deep golden brown and jammy. This can take 25–30 minutes, but trust me, it’s worth every second.
2. Whisk the Custard
Whisk together the eggs, milk, cream, Dijon, garlic powder, and seasonings until smooth.
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3. Tear & Layer
Tear croissants into bite-sized chunks and layer them in a greased 9x13” dish. Scatter caramelized onions and shredded Gruyère throughout. Pour the custard over the croissant mixture and gently press everything down so it soaks evenly.
4. Chill, Top & Bake
Let the casserole rest for at least 30 minutes—or overnight. Then top with the remaining cheese and prosciutto. Bake until puffed and golden on top and just set in the center.
Chef’s Tips & Tricks
- Use day-old croissants: They soak up the custard better than super fresh ones. If you only have fresh croissants you can toast them in the oven at 400F for 5-7 minutes while the onions cook.
- Make it ahead: Assemble the night before and bake fresh in the morning.
- Double the onions: They freeze well, and you’ll want extras for sandwiches or pizza later!
Pair This Casserole With…
- A crisp arugula salad with lemon vinaigrette
- Fresh berries or fruit salad
- A splash of sparkling wine—or a strong cup of coffee
- Apple Pecan Salad
- Smoked Chicken Harvest Salad
- Crispy Smashed Potatoes
- Flat Iron Steak
How to Store & Reheat
- Store: Leftovers keep well in an airtight container in the fridge for up to 4 days.
- Reheat: Cover and warm in a 325°F oven for 10–15 minutes.
- Freeze: You can assemble the unbaked casserole, wrap it tightly, and freeze. Thaw overnight in the fridge before baking.
Recipe
Caramelized Onion & Gruyère Croissant Casserole
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*Easily adjust serving size to your liking by clicking the number under servings or with the boxes below in the ingredients section.
Equipment
- 9X13 casserole dish
- Mixing bowl
- Whisk
Ingredients
For the caramelized onions:
- 2 tablespoons olive oil or butter, divided for onions and greasing the baking dish
- 2 large yellow onions thinly sliced into half moons
- 1 teaspoon kosher salt
- water as needed
- 1 tablespoon balsamic or sherry vinegar optional
For the custard:
- 7 large eggs
- 2 cups whole milk
- ½ cup heavy cream
- 1½ teaspoons Dijon mustard such as Grey Poupon
- ½ heaping teaspoon garlic powder
- ½ teaspoon dried thyme
- ¾ teaspoon kosher salt
- ½ teaspoon freshly cracked black pepper
For the casserole:
- 4 large croissants preferably day old, torn into 1½-inch pieces (see notes if using fresh croissants)
- 2 heaping cups shredded Gruyère cheese (about 15 ounces) fontina or Swiss cheese
- 5 strips proscuitto torn into bite-sized pieces, optional
- For serving: fresh thyme optional
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Instructions
- Make the caramelized onions: In a large skillet over medium heat add the sliced onions, salt, and olive oil. Cook slowly, stirring often, (make sure to stir in the dark bits from the bottom of the pan and deglaze with water as needed) for about 25–30 minutes until they’re deep golden and jammy. Stir in the sherry vinegar for brightness. Set aside to cool. While the onions cook, make the custard and grate the cheese. (See notes if using fresh croissants.)
- Make the custard: In a mixing bowl, whisk together the eggs, milk, cream, Dijon, garlic powder, dried thyme, salt, and pepper. Set aside.
- Prep baking dish: Grease a 9x13-inch baking dish with olive oil or softened butter. Scatter torn croissants in the base, then top with the caramelized onions and 1 cup of the shredded Gruyère. Pour the custard evenly over the croissant mixture. Gently press down to help the bread soak it up.
- Rest: Cover and refrigerate for at least 30 minutes or overnight.
- Bake: Preheat the oven to 350°F (175°C). Sprinkle the top with the remaining Gruyère and prosciutto (if using, torn into small pieces). Bake uncovered for 40–45 minutes, until puffed, golden, and just set in the center. Let rest for 10 minutes and top with fresh thyme just before serving, if desired.
Gail Lobsinger
I made this recipe for a brunch with my cousins. Everyone loved the flavors of the caramelized onions, gruyere cheese and prosciutto with the custard croissant base. I will be making this recipe again. Thanks Suzie
Suzie J.
Thank you, Gail! I appreciate you trying my recipe, and I'm so glad everyone loved it. Xo, Suzie