Place croutons on the bottom of a greased 2-quart casserole dish and top with 1 1/2 cups shredded cheddar cheese.
Brown sausage in a skillet; drain. Spread on top of the cheese.
With an electric mixer, beat eggs with 2 1/2 cups milk, dry mustard, salt, and pepper; pour over sausage and refrigerate mixture overnight.
In a bowl, combine cream of mushroom soup and 1/2 cup milk. Pour into the casserole dish and sprinkle with 1/2 cup shredded cheddar cheese.
Bake at 300° F for 1 1/2 hours.Servings: 6