This recipe should make 12 muffin size frittatas.
Spray muffin tins liberally with nonstick spray. Whisk the eggs, milk, pepper, and salt in a large bowl to blend well. Combine extra ingredients with eggs or add a small amount of extra ingredients to each cup. Fill prepared muffin cups almost to the top with the egg mixture. (Eggs should be evenly distributed.) Bake until the egg mixture puffs and is just set in the center, about 12 to 14 minutes for regular sized muffins. Using a rubber spatula, loosen the frittatas from the muffin cups and slide the frittatas onto plates. Serve immediately.
This recipe can be made in a large skillet, started on the stovetop, and transferred to the oven (if skillet is oven-safe) to finish cooking. lt will take a little longer to cook through.