Brown sausage in a large skillet over medium-high heat; drain fat.
Place croutons in a 9X13 inch baking dish.
Sprinkle cheese over croutons; top with browned sausage. Beat eggs and 2 1/2 cups of milk in a mixing bowl, and pour over the casserole. Cover baking dish, and refrigerate overnight.
The following morning, preheat oven to 300 degrees F (150 degrees C).
Combine the mushroom soup, ground mustard, and the remaining 1/2 cup milk; pour over the top of the casserole.
Bake, uncovered, until browned and set, about 1 1/2 hours.
