Our Test Kitchen has made lots and lots of breakfast casseroles for many years, but this croissant breakfast casserole is a preferred still. Made with day-old mini croissants, caramelized onions, and an abundant and cheesy egg custard that is seasoned with a touch of Dijon, this croissant breakfast casserole is a real crowd-pleaser.

Put together the casserole and shop it, unbaked, in the refrigerator overnight. Keep an eye on the casserole as it bakes.
Ingredients for Croissant Breakfast Casserole
To make this casserole, you’ll require:

Ham steaks: Chopped and browned for a meaty bite. Alternative with collapsed bacon or browned sausage, if chosen.
Olive oil: For cooking the ham, onions, and spinach.
Vidalia or other sweet onions: Caramelized for an abundant, sweet flavor in the casserole.
Infant spinach: Wilted into the mix for color and freshness.
Big eggs: Helps bind and set the casserole.
Entire milk: The liquid base of the casserole filling. Other types of milk can be used if required, or a mix of 2 percent milk and half-and-half.
Dijon mustard: Adds a tasty flavor that contrasts the ham, onions, and cheese.
Kosher salt and black pepper: Enhances the flavor.
Fontina cheese: A nutty, buttery taste. Can be replaced with other shredded cheeses, consisting of Cheddar, gruyere, gouda, or provolone.
Day-old mini croissants: Can substitute with regular croissants (if big, cut into chunks), whether day-old or fresh, in addition to cubed brioche, challah, or other kinds of enriched breads.
How To Make Croissant Breakfast Casserole
Full guidelines are below, however here’s a quick recap before you get going:

Step 1. Coarsely chop ham and cook with 1 tablespoon of oil until fat is rendered. Transfer to a bowl, reserving drippings in frying pan. Add onions and cook until deeply browned, about 30 minutes. Add spinach; cook until wilted. Include mix to bowl with ham; cool 10 minutes.
Stir into ham along with 1 cup of cheese. Organize croissants in a lightly greased 11- x 7-inch baking meal; cover with egg mix and staying cheese.
Step 3. Preheat oven to 375 ° F. Uncover casserole and put on a flat pan. Bake till golden brown, about 1 hour. Guard with foil after 25 minutes, if required, to prevent excess browning.
What To Serve with Croissant Breakfast Casserole
This meaty, tacky casserole is an ideal dish to put on the table for a large family-style breakfast or brunch. Serve with a side of fresh fruit salad, side salad, heirloom tomatoes, crispy hash browns, or something sweet like warm cinnamon rolls for a hearty meal everyone will remember.

How To Store and Reheat Leftover Croissant Breakfast Casserole
Cool remaining croissant casserole for as much as 3 days, or freeze for as much as three months. Thaw over night in the fridge if frozen, and reheat in a preheated 350ºF oven till hot throughout before serving.