Preheat broiler. Off heat, coat same skillet with cooking spray. Arrange bread slices tightly in bottom of skillet. In a large bowl, whisk together eggs, egg whites, milk, mustard, salt and pepper; pour over bread and allow milk mixture to soak in entirely (like when you make French toast). Sprinkle bread with 1 tablespoon of Parmesan cheese. Place skillet over medium to medium-high heat. Cook, shaking pan frequently to promote even cooking, until almost cooked through to the top, about 4 or 5 minutes.