Preheat your oven to 350°F and lightly grease a standard muffin tin with avocado oil.
Finely chop the fresh baby spinach and the red bell pepper into small, uniform pieces.
In a large mixing bowl, whisk together the whole eggs, egg whites, sea salt, black pepper, and garlic powder until the mixture is well combined and slightly frothy.
Distribute the chopped spinach and red bell pepper evenly across six of the muffin cups.
Carefully pour the egg mixture into each cup, filling them about three-quarters of the way to the top.
Sprinkle the crumbled feta cheese over each cup to add a salty, creamy finish.
Place the tin in the oven and bake for 20 to 25 minutes, or until the eggs have set and the tops are beginning to turn golden brown.
Remove from the oven and let the egg cups cool for at least 5 minutes before using a thin spatula to gently pop them out of the tin.