In a large mixing bowl, combine eggs, milk, mustard, and salt. In a 9x13 baking dish spread butter along bottom. Place 6 slices of bread on top of butter. Sprinkle bread with pepper. Layer shredded cheese then the cooked sausage. Pour egg mixture evenly over casserole.
Bake uncovered for 55-60 minutes or until eggs are set. Tent with foil if top begins to brown too quickly.
Can be made the night before and heated in the morning.