1 slice mozzarella cheese, cut diagonally into quarters
4 slices Roma tomato (optional)
Coat a small nonstick saucepan with cooking spray; heat over medium heat. Sprinkle pepperoni, sweet pepper and mushrooms into saucepan; cook for 2 minutes.
In a small bowl, whisk together egg whites and milk; pour over pepperoni mixture in saucepan. Cook until egg white mixture begins to set. Using a spatula, fold the partially cooked egg white mixture over; cook about 2 minutes more or until it is cooked through.
Meanwhile, toast the deli flat halves. Place cut sides down. While still warm, spread each half with 1 teaspoon of the pizza sauce; place two of the cheese quarters on each half.
Spoon half of the egg mixture over each of the prepared deli flat halves. If desired, top with tomato.