Place croutons in a greased 9x13 inch baking dish. Sprinkle 1 1/2 cups cheese on top. Spread sausage on top of cheese.
Beat eggs with 2 1/4 cup milk and dry mustard. Pour over croutons and refrigerate overnight.
Remove refrigerated dish and sprinkle remaining shredded cheese on top.
In a separate bowl, dilute soup with remaining 1/2 cup milk and pour over top.
Bake at 325 for 1 1/2 hours.