Preheat oven to 300°F (150°C) and spray a (silicone) muffin pan with cooking spray or grease with oil.
Optional step — Place a rack in the center of the oven and add a second rack beneath it. Fill a large baking pan with water and place it in the lower rack to create steam.
Add to a blender liquid egg whites, cottage cheese, shredded mozzarella, salt, garlic powder and pepper. Blend until smooth.
After blending, use a fork or spoon to mix in chopped spinach and diced red pepper. Do not blend again.
Fill ¾ of each muffin mold, distributing the egg white mixture evenly. Sprinkle with more diced red pepper and chopped spinach on top, if you desire.
Bake in the middle rack of the oven for 20 to 25 minutes, or until they have set in the center. Let cool before serving.