Spray 9x13 dish with nonstick cooking spray. Remove the crusts from the bread slices and spread 1 side of the bread in butter. Arrange butter side down in the prepared baking dish.
Brown the sausage in a skillet, stirring until crumbly; drain. Sprinkle the sausage, mushrooms, onion and bell peppers evenly over the bread.
Beat the eggs in a bowl. Add the milk, dry mustard, salt and pepper and mix well. Pour over the layers and sprinkle with the cheese. Chill, covered for 8 to 12 hours.