Cook the vegetables first: Sautéing the spinach, bell pepper, and onion before baking helps remove extra moisture and gives the frittata better flavor.
Whisk the egg whites well: Mixing them thoroughly with the milk and seasonings helps the frittata bake up more evenly.
Do not over bake: Bake just until the center is set so the frittata stays tender and does not turn rubbery.
Let it cool slightly before slicing: This helps it hold together better and makes it easier to cut clean slices.
Great for meal prep: Slice it into portions once it cools so you can easily reheat servings throughout the week.