Trim 10-12 pieces of bread and place in a well-greased 9x13 inch glass baking dish. Place layer of thinly sliced American cheese on bread slices. Fry 1 pound of sausage, crumble, drain and sprinkle over cheese and bread.
Beat 6 eggs, 3 1/2 cups milk, 1/4 teaspoon dry mustard and 1/2 teaspoon salt. Pour half of the egg mixture over ingredients in pan and repeat process. Cover with foil and let stand overnight in the refrigerator. Bake 1 hour at 325 degrees. Uncover last 10 minutes of baking. Let stand 10-15 minutes before cutting.