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Breakfast Casserole

Breakfast casserole with caramelized onion jam, torn croissant chunks, and a bubbly Gruyère cheese crust. Slow-cooked onions create deep, jammy ribbons through the cut edge, while eggs and milk bake into a puffed, golden-bronze top.
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Prep Time 1 hour 15 minutes
Cook Time 47 minutes
resting 2 hours
Total Time 4 hours 2 minutes
Servings: 8 servings
Course: Breakfast
Cuisine: French
Calories: 470
Ingredients Equipment Method Notes

Ingredients
  

croissants
  • 4 croissants torn into chunks
yellow onions
  • 4 yellow onions thinly sliced (caramelize until jammy)
butter
  • 2 tbsp butter for caramelizing onions
beef or chicken broth
  • 1 cup beef or chicken broth for deglazing and soaking
eggs
  • 6 eggs
whole milk
  • 1.5 cup whole milk
Gruyère
  • 1 cup Gruyère divided; 1 cup for sprinkling, plus remaining for topping
Swiss
  • 0.5 cup Swiss shredded
thyme
  • 1 tbsp fresh thyme plus extra for garnish
Dijon mustard
  • 1 tsp Dijon mustard
salt
  • 0.25 tsp salt to taste; season the egg mixture
black pepper
  • 0.25 tsp black pepper to taste; season the egg mixture
fresh thyme
  • 1 fresh thyme to garnish

Equipment

  • 1 sheet pan
  • 1 9x13 baking dish

Method
 

Caramelize the onion jam
  1. Melt butter in a skillet over medium-low heat and add sliced onions, cooking 40–45 minutes until deep golden and jammy, stirring occasionally and keeping the heat low to avoid burning.
  2. Add broth to the pan and deglaze, scraping up browned bits until the mixture is smooth and glossy, about 1–2 minutes.
Assemble the casserole
  1. Grease a 9x13 baking dish, then layer torn croissant chunks in an even layer.
  2. Spread the caramelized onion jam throughout the croissants, aiming for a visible ribbon-like layer across the dish.
  3. Sprinkle 1 cup Gruyère evenly over the onions.
Make the egg custard
  1. In a bowl, whisk eggs, milk, Dijon mustard, fresh thyme, salt, and black pepper until fully combined and lightly foamy.
  2. Pour the egg mixture over the croissant-onion layer so it soaks down into the chunks.
  3. Top with the remaining Gruyère and the shredded Swiss to cover the surface.
Chill
  1. Refrigerate the casserole 2+ hours or overnight, uncovered if possible, until the custard sets slightly and the croissants absorb the liquid.
Bake and finish
  1. Bake at 350°F for 45 minutes until puffed and deeply golden across the top.
  2. Broil for 2 minutes to intensify browning and form a deeper cheese crust.
  3. Garnish with fresh thyme right before serving for a fresh, aromatic finish.

Notes

Pro tip: don’t rush the onion caramelization—45 minutes is the minimum for true jamny sweetness and that French onion soup flavor. Refrigerate leftovers up to 3 days; reheat covered at 325°F until warmed through. Freezing is not recommended because the croissant texture can soften after thawing. For a lighter option, use whole milk or a 2% dairy alternative and reduce the cheese slightly without changing the bake time.