Combine egg, egg whites and water in a bowl, season with salt and pepper and whisk well.
Add spinach to egg mixture.
Spray a nonstick skillet with cooking spray.
Add eggs and cook for 3-4 minutes until eggs begin to set, using a spatula, gently pull in the sides of the omelet to let the uncooked egg run to the edges of the pan.
Place cheese on one side of the omelet and gently fold in half.
Allow to cook for 2 minutes or until cheese is melted.