Preheat oven to 375. Coat a 7 x 11 inch baking dish with nonstick spray.
Brown sausage in saute pan over medium high heat til almost fully cooked, 5 minutes. Add onion and saute til softened, 3 minutes. Transfer sausage mixture to paper towel lined plate.
Whisk together eggs, milk, mustard, sage and salt. season with pepper.
Arrange bread in bottom of prepared baking dish. Top bread with apple and 1 1/2 cups cheese, then add sausage mixture. Pour egg mixture over sausage, taking care it doesn't spill over, then top with remaining 1/2 cup cheese.
Bake casserole til cheese is browned and eggs are set in the center and cooked through, about 45 minutes or to 160 degrees. Serves 8