Mix croutons, Cheddar cheese, and bacon together in a bowl; transfer to an 11x7-inch baking dish.
Beat eggs, mustard, onion powder, and black pepper together in a bowl; pour over croutons mixture. Cover dish with plastic wrap and refrigerate, 8 hours to overnight.
Preheat oven to 350 degrees F (175 degrees C). Remove plastic wrap from dish.
Bake in the preheated oven until a knife inserted near the center comes out clean, about 45 minutes. Let casserole stand for 5 minutes before serving.
