Preheat oven to 325 degrees. Line a muffin tin with foil liners.
Fill each well in the muffin tin with a small amount of peppers, scallions and cheese. In a separate bowl (I used a 4-cup measuring cup for this, as it had a handy spout for pouring), whisk together the eggs, egg whites and milk. Whisk in the salt, garlic powder and pepper. Divide egg mixture evenly among muffin wells, filling 2/3 to 3/4 of the way full.
Bake for 30-35 minutes or until golden and eggs are set. Allow to cool for about 5 minutes in the tin before transferring to a wire rack to cool completely.
Store in a zip-top bag for 3-5 days in the refrigerator or for up to 1 month in the freezer.
To reheat, remove the foil liner (...it seems automatically intuitive but sometimes in the morning it is not), wrap in a paper towel and microwave on high for 30 seconds to one minute.
Notes
I used scallions and green pepper in my muffins, but you could use a comparable amount of any vegetable that you like. Make sure to quickly blanch any veggies such as broccoli beforehand.