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. Add peppers and onions and sauté until softened, about 5 minutes, stirring occasionally. Add spinach and stir gently until wilted. Remove from heat.
In a large bowl, whisk together eggs, milk, cheese, salt and pepper. Pour egg mixture into prepared muffin tins so that each one is about ½ to 2/3 full. Add sautéed vegetables evenly to the egg mixture.
Bake for 20-25 minutes or until puffed up and golden brown. Leave in muffin tin for about 5 minutes before removing and serving.