Preheat oven to 250 degrees F. Line a large jelly roll pan with foil. Divide sausage into small (1/2 inch) pieces and place on foil lined pan. Bake for 30 minutes until sausage is cooked. Drain sausage on paper toweling.
Grease a 9x13-inch pan with butter. Set aside.
Trim crusts from bread and cut into 1-inch cubes. Set aside.
Beat eggs until frothy in a large bowl. Add milk, salt, pepper and mustard to eggs. Beat well. Add cheese and stir. Add cubed bread to egg mixture. Stir until bread is just damped. Stir in sausage.
Pour mixture into prepared baking pan. Cover and refrigerate for a minimum of 2 hours. (Overnight is best.)
Remove baking pan from refrigerator and uncover. Place rack in the middle position in the oven. Place baking pan in cold oven. Preheast oven to 350 degrees F. Do NOT start timer until the oven has reached 350 degrees F. Bake for 55 minutes.
Allow pan to rest for 10 minutes. Spoon on to serving dishes.