The night before, cook sausage and onion over medium-high heat in a nonstick skillet until sausage is no longer pink. Drain and cool.
In a large bowl, beat the eggs, egg whites, milk, mustard, salt and pepper. Stir in the bread cubes, cheese and sausage mixture. Pour into a 13 x 9 inch baking dish coated with cooking spray. Cover and refrigerate overnight.
Remove from the refrigerator 30 minutes before baking. Bake, uncovered, at 350 degrees F for about 35 to 40 minutes or until a knife inserted near the center comes out clean. Let stand for 5 minutes before cutting. Enjoy!
Serving Size: 1 slice • Calories: 211 • Fat: 11 g • Carbs: 13 g • Fiber: 1 g • Protein: 15 g