Caramelized Onion & Gruyère Croissant Casserole
Golden, buttery croissants meet melty Gruyère and sweet, jammy onions in this savory make-ahead brunch bake. Perfect for holidays or special occasions.
Click the stars to rate.
*Easily adjust serving size to your liking by clicking the number under servings or with the boxes below in the ingredients section.
Equipment
- 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
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.
Notes
No day-old croissants? No problem! If you only have fresh croissants you can toast them in the oven at 400F for 5-7 minutes while the onions cook.
Nutrition
Calories: 323kcalCarbohydrates: 19gProtein: 11gFat: 23gSaturated Fat: 10gPolyunsaturated Fat: 2gMonounsaturated Fat: 8gTrans Fat: 0.02gCholesterol: 190mgSodium: 747mgPotassium: 248mgFiber: 1gSugar: 8gVitamin A: 746IUVitamin C: 2mgCalcium: 129mgIron: 1mg
Tried this recipe?Let us know how it was!