Make the croque monsieur casserole

Cup of Yum
1. Butter a 9-by-13-inch (23-by-33-cm) casserole dish.
2. Slice the croissants lengthwise and spread with a bit of mustard.
3. Tuck 2 slices of ham and about 1/4 cup cheese inside each croissant.
4. Cut each croissant into quarters, and fit the pieces in the dish. Turn them different ways, so some have frilly slices of ham showing.
5. In a large bowl, whisk the eggs, then pour in the milk and whisk to combine. Season lightly with salt and pepper.
6. Evenly pour the egg mixture over the croissant pieces. Press down with your hands to make sure all the liquid is absorbed. Cover with plastic wrap and refrigerate overnight.

Make the béchamel sauce

1. In a medium saucepan over medium heat, warm the milk just until a skin begins to form on top, but the milk isn't boiling.
2. Meanwhile, in another medium saucepan over medium heat, melt the butter. Add the flour and stir constantly until the mixture bubbles, about 2 minutes.
3. Slowly pour the hot milk into the pan with the flour-butter mixture, whisking the whole time. Bring the mixture to a boil, then reduce to low and simmer until the béchamel thickens, 2 to 3 minutes more. Season with nutmeg, salt, and pepper.
4. Pour the sauce into a container, press plastic wrap against the surface to prevent a skin from forming, and let cool to room temperature. Refrigerate overnight.

Assemble the casserole

1. The next morning, remove the casserole and béchamel sauce from the refrigerator 30 minutes before baking. Crank the oven to 350°F (175°C).
2. Uncover the casserole and place it on a foil-lined rimmed baking sheet. Using a whisk, stir the béchamel to loosen it. Spread the sauce over the top, and sprinkle with the remaining cheese.
3. Bake the casserole until puffed and golden brown, 50 to 55 minutes.
4. Serve immediately.