
Preheat oven to 375°F. Grease or spray a tin for 12 muffins.
In a medium frying pan, heat the oil over medium high heat. When hot, add peppers and onions and saute until softened, about five minutes, stirring occasionally. Add spinach and stir gently until wilted. Remove from heat.
Meanwhile, in a large bowl, whisk together eggs, egg whites, skim milk, cheese, salt and pepper.
Pour egg mixture into prepared muffin tin so that each one is about 1/2 to 2/3 full. Add sauteed vegetables evenly to each of the 12 muffins.
Bake for 20-25 minutes or until puffed up and golden brown. Leave in muffin tin for about 5 minutes before removing and serving.