Preheat the oven:
Preheat your oven to 375°F (190°C). Grease a 12-cup muffin tin generously with nonstick spray or olive oil.
Whisk the eggs:
In a large bowl, whisk together the eggs, milk, salt, and pepper until smooth and airy.
Add fillings:
Evenly distribute your choice of fillings (vegetables, cheese, or meats) into each muffin cup.
Pour the egg mixture:
Carefully pour the egg mixture over the fillings, filling each cup about three-quarters full.
Bake:
Bake for 18–20 minutes, or until the muffins are puffed up and set in the middle.
Cool and store:
Allow to cool for 5 minutes before removing from the tin. Store leftovers in the fridge for up to 5 days or freeze for up to 2 months.