Prepare your muffin pan by greasing it generously with butter or coating it with cooking spray; set aside.
Take a large bowl and whisk the eggs, egg whites, Italian seasoning, black pepper powder, and salt until combined well; cover and set aside.
Now, divide the spinach, red & green bell peppers, and tomatoes among the muffin cups filling, the muffin cups will be 2/3 full)
Whisk the egg mixture again, and pour it carefully in the muffin cups filling almost to the top.
Sprinkle the crumbled feta evenly on the top.
Air Fryer at 8 to 10 minutes, at 330 degrees F, or until they’re golden brown and appear set. Let them cool in the pan for 5-10 minutes and then run a butter knife around the edges to loosen them.
Remove from the pan and serve immediately or freeze for later use.