Preheat the oven to 350°F (175°C). Grease a 12-cup muffin pan well with cooking spray or oil.
In a large mixing bowl, whisk together the eggs, egg whites, kosher salt, dried basil, dried oregano, and black pepper or cayenne.
Divide the chopped spinach, diced red bell pepper, diced green bell pepper, and quartered cherry tomatoes evenly between the muffin cups.
Pour the egg mixture over the vegetables, filling each cup about 3/4 full.
Sprinkle crumbled feta cheese over the top of each cup. Add a little extra feta if desired.
Bake for 18 to 22 minutes, or until the egg muffins are puffed, set in the center, and lightly golden on top.
Let cool in the pan for 5 minutes.
Run a small knife or silicone spatula around the edges, then remove the muffins from the pan.
Serve warm with avocado, salsa, hot sauce, or freshly chopped parsley.