Preheat oven to 180 degrees. Grease nonstick muffin pan with cooking oil spray.
In a large bowl, add egg whites, spinach, peppers, mushrooms, 2/3 of the cheese, scallions, salt and pepper. Whisk until evenly combined.
Using a 1/3 cup, scoop out batter and pour into muffin cavities. You should have enough batter and ingredients to make 10 muffins. Sprinkle remaining cheese over the surface of the muffins.
Bake for about 20-22 minutes or until muffins have puffed up and egg whites are completely cooked. Muffins will sink back down as they cool.
Muffins are best eaten warm. They can be stored in the fridge or freezer.