Lightly coat a 9 x 13-inch baking dish with cooking spray.
Slice the croissants, spread each one with mustard, and fill them with the ham and cheese. Shingle the croissant sandwiches into the casserole dish.
Beat the eggs in a medium bowl, add the milk, salt, and pepper, and mix well. Pour over the sandwiches and cover the dish with foil. Weight the top with a plate or cookie sheet to submerge the sandwiches in the egg mixture and refrigerate for at least 4 hours or overnight.
Preheat the oven to 350 degrees F. Bake, covered with the foil, for 30 minutes. Remove the foil and bake for an additional 30 minutes. Serve.
Notes
It’s all about the croissants! If you can find some at a local bakery, the superior quality and freshness will certainly shine through.
Variations: Spice the sandwiches up with pepper jack cheese or add a touch of sweetness with honey Dijon mustard.