Croissant Breakfast Casserole with Caramelized Onions | CookOS

Croissant Breakfast Casserole

Name: Croissant Breakfast Casserole with Caramelized Onions
Prep time: 25 min
Cook time: 1 h
Total time: 1 h 30 min
Yield: 4 servings
Ingredients: 1 tsp vegetable oil; 1 medium yellow onion, halved and thinly sliced; 3 large croissants, cut into 1″ pieces; 2 large eggs; 1 cup grated fontina or gruyere; 1 tsp dijon mustard; 1 ½ cups whole milk

This croissant breakfast casserole combines buttery croissant pieces with a rich custard filling, sharp fontina cheese, and slowly caramelized onions for a sophisticated morning dish. A touch of Dijon mustard adds subtle tanginess that elevates the savory-sweet flavor profile.

🥗Vegetarian 🏷️Breakfast 🏷️Casserole 🏷️Croissant 🏷️Slow cook Family-friendly

About·Privacy·Terms

🏷️

Total

1h 30m

Servings

Difficulty

Medium

Est. Cost

$16–$26

Recipe Source

View original recipe

From Recette Magazine

Ingredients

1 tsp vegetable oil
1 medium yellow onion, halved and thinly sliced
3 large croissants, cut into 1″ pieces
2 large eggs
1 cup grated fontina or gruyere
1 tsp dijon mustard
1 ½ cups whole milk

7 Ingredients

Delivery in as fast as one hour.^*

Prices vary by store

^*Service fees apply. Instacart^® and the Instacart logo are registered trademarks of Maplebear Inc. d/b/a Instacart.

Instructions

1
Heat 1 tsp vegetable oil in a medium nonstick skillet over medium-high heat until shimmering. Add onions and ¼ tsp salt. Reduce heat to medium-low and cook, stirring occasionally until onions are browned, about 10 minutes.
2
Add 1 tbsp water to skillet and stir until water evaporates. Repeat once more until onions are soft and brown.
3
In a large bowl whisk together eggs, dijon, milk, and ½ tsp salt. Spray a Suvie pan with nonstick cooking spray and arrange croissants in the pan. Pour egg mixture over croissants. Stir in onions and cheese until will incorporated.
4
Insert pan into your Suvie, input settings, and cook now or schedule. Bottom Zone: Slow Cook Low for 1 hour
5
After the casserole has finished cooking, broil 5 minutes if desired, until golden.
6
Remove from Suvie and let cool 5 minutes before slicing and serving.

Was this recipe helpful?

Tips & Notes

Pro tips

Caramelize onions low and slow over 25–35 minutes—rushing this step results in merely softened, not deeply sweet onions.
Cut croissants into roughly 1-inch pieces; uneven sizes are fine and create pockets of varying texture in the final bake.
Whisk eggs, milk, Dijon, and salt together before pouring; this ensures even custard distribution and prevents streaky patches.
Let the assembled casserole rest 15 minutes before baking to allow the croissants to absorb some custard, which prevents a dry top.

Substitutions

Gruyere or Emmental → Aged cheddar or Jarlsberg (maintains nutty, sharp flavor; avoid soft cheeses)
Dijon mustard → Whole grain mustard (coarser texture; use same quantity for similar tang)
Whole milk → Half-and-half (richer custard; reduce quantity to 1 1/4 cups to prevent over-softness)

Storage & make-ahead

Cover and refrigerate leftovers up to 3 days. Reheat in a 325°F oven until warmed through, 15–20 minutes. Freezing is not recommended due to custard texture loss.

Common Questions

Can I make this casserole the night before?

Yes. Assemble the casserole, cover, and refrigerate overnight. Bake the next morning, adding 5–10 extra minutes to the bake time if starting from cold.

What cheese works if I don't have fontina or gruyere?

Emmental, Jarlsberg, or aged cheddar work well. Avoid fresh mozzarella, which will become stringy; choose a firm, nutty-flavored cheese.

How do I know when the casserole is done baking?

The custard should be set but still slightly jiggly in the very center, and the top should be golden. A knife inserted off-center should come out clean.