Preheat your oven to 350°F and lightly grease a standard muffin tin with the olive oil.
Finely chop the fresh spinach, red bell pepper, and red onion into small, uniform pieces.
In a large mixing bowl, whisk together the whole eggs and liquid egg whites until the mixture is well combined and slightly frothy.
Stir the chopped spinach, peppers, and onions into the egg mixture.
Add the sea salt, black pepper, garlic powder, and smoked paprika, whisking again to distribute the spices evenly.
Pour the egg and vegetable mixture into 6 of the muffin cups, filling each one about three-quarters of the way to the top.
Sprinkle the crumbled feta cheese evenly over the top of each muffin cup.
Bake for 20 to 25 minutes, or until the eggs are firmly set and the tops have turned a light golden brown.
Remove from the oven and let the muffins cool in the tin for 5 minutes before using a silicone spatula to gently pop them out.