Preheat oven to 400 degrees. Clean mushrooms and scoop out the insides and stem with a spoon. Place on baking pan and bake for 7 minutes. While mushrooms are baking, grate Mozzarella, and combine in a bowl with one egg white. (Usually low-fat cheese has trouble melting, so this technique helps the cheese melt while adding a bit of protein.) Remove mushrooms from oven, and turn the broiler to High. On the mushrooms layer pizza sauce, cheese mixture, olives, and pepperoni slices. Return pan to oven and broil on High for 3 minutes or until cheese has melted and started to brown.