Description

This Overnight Croque Monsieur Casserole is a decadent, make-ahead breakfast or brunch dish featuring layers of buttered bread, Canadian bacon, caramelized onions, Gruyere cheese, and a rich egg custard. The casserole soaks overnight for deep flavor infusion and bakes to a golden, cheesy perfection, garnished with fresh thyme and Parmesan cheese. It’s a delightful twist on a classic French sandwich, served easily from a single baking dish.

Ingredients

Scale

For the Casserole:

1 Loaf Martin’s Old-Fashioned Real Butter Bread, stale or toasted, cut into thick cubes
1/2 Pound Canadian Bacon, cut into 1/2-inch dice
2 Cups Gruyere Cheese, shredded (about 4.5 ounces)
1/4 Cup Parmesan Cheese, finely grated (about 1 ounce)

For the Onion-Garlic Mixture:

1 Tablespoon Unsalted Butter, plus more to coat pan
1 Yellow Onion, roughly chopped (about 1 1/2 cups)
4 Cloves Garlic, thinly sliced
Salt and Pepper, to taste
1 Tablespoon Fresh Thyme, roughly chopped, plus 1 teaspoon for garnish

For the Egg Custard:

8 Large Eggs
1 1/2 Cups Half-and-Half
2 Tablespoons Dijon Mustard
1/2 Teaspoon Hot Sauce, plus more for garnish
1 1/2 Teaspoons Kosher Salt

Instructions

Prepare the Baking Dish: Coat a 3-quart (13×9-inch) baking dish with butter and set aside to prevent sticking.
Sauté Onion and Garlic: Heat a large frying pan over medium heat and add butter. Once slightly frothy, add the chopped onion, season with salt, and cook for 3 to 4 minutes until softened. Add garlic, season with salt and pepper, and cook for about 1 minute until fragrant. Remove from heat and stir in fresh thyme.
Make the Egg Custard: In a large bowl, whisk together eggs, half-and-half, Dijon mustard, hot sauce, and kosher salt until the mixture is smooth and evenly combined.
Assemble the Casserole: Arrange half of the bread cubes in the bottom of the buttered dish. Sprinkle with 1 cup of Gruyere cheese, half of the onion-garlic mixture, and half of the diced Canadian bacon. Repeat the layering with the remaining bread, onion mixture, bacon, and Gruyere. Pour the egg custard evenly over the entire casserole, pressing the bread gently to submerge it fully.
Refrigerate Overnight: Cover the assembled casserole with plastic wrap and place a heavy plate on top to weigh down the bread, ensuring it soaks in the custard. Refrigerate for 3 to 12 hours.
Bake the Casserole: When ready to bake, remove the plastic wrap and weight. Cover the dish with aluminum foil and place it on the middle rack of the oven. Set the oven to 325°F (no preheating required) and bake for 35 minutes.
Finish Baking and Broil: Remove the foil and increase the oven temperature to 375°F. Sprinkle the Parmesan cheese over the casserole and bake for another 10 to 15 minutes until the top is golden brown and bubbly.
Serve: Remove the casserole from the oven, sprinkle with the remaining fresh thyme, and allow it to cool for at least 5 minutes. Serve with extra hot sauce on the side for added heat, if desired.

Notes

Using stale or toasted bread helps the casserole absorb the custard better without becoming too soggy.
The casserole can be assembled up to 12 hours in advance, making it perfect for busy mornings or entertaining.
If desired, substitute Canadian bacon with ham or cooked bacon strips.
Adjust the amount of hot sauce in custard and as garnish to suit your spice preference.
Ensure the casserole cools slightly before serving to allow it to set for cleaner slices.
This dish can be reheated in the oven at 350°F for 10-15 minutes if served later.

Prep Time: 15 minutes
Cook Time: 50 minutes
Category: Breakfast Casserole
Method: Baking
Cuisine: French-American

Keywords: Croque Monsieur, breakfast casserole, brunch recipe, overnight casserole, cheesy breakfast bake, Swiss cheese, Canadian bacon, make-ahead breakfast