Preheat the oven to 375°F (190°C) and grease a 12-cup muffin tin with cooking spray or butter.
Whisk the eggs in a large mixing bowl, adding milk, salt, pepper, and garlic powder (if using) until well combined.
Chop vegetables, shred cheese, and cook meats if needed.
Distribute the fillings evenly into each muffin cup, then pour the egg mixture over them, filling each cup about 3/4 full.
Bake for 18-22 minutes, or until the egg muffins are set and lightly golden on top.
Let the muffins cool in the pan for 5 minutes before removing them with a knife or spatula. Serve immediately, or store for later use in the refrigerator or freezer.