Preheat the oven to 350 degrees Fahrenheit.

Heat the avocado oil in a large skillet over medium-high heat. Add the onion and saute for 2 to 3 minutes, until the onion begins to soften. Add in the bell pepper and spinach and continue cooking and stirring until spinach has wilted. Stir in the chopped chicken and sun-dried tomatoes.

Spray the holes of a muffin tin with cooking spray (this makes it easy to remove the egg muffins from the muffin pan after baking).

Divide the spinach and chicken mixture evenly among the 12 muffin holes.

In a bowl or measuring cup, whisk together the eggs and egg whites. Add any seasonings you like here, such as dried herbs or salt. I like using a mixing bowl that has a pour spout to make it easy to pour the egg mixture. A large measuring cup (3 cups or larger) will work too.

Pour the beaten eggs into the muffin holes, filling the muffin holes most of the way up. Sprinkle the tops with feta cheese. 

Bake on the center rack of the preheated oven for 20 to 25 minutes, or until the muffins are cooked through.

If you have a meat thermometer on hand, you can check the internal temperature of the muffins using a thermometer. They are fully cooked once they reach a minimum of 190 degrees Fahrenheit.

Allow the mini frittatas to cool completely. If you try releasing the muffins from the tray before they are entirely cooled, some of the muffin will stick to the bottom of the pan. Once cooled, run a butter knife between the sides of the muffin tray and the egg muffins to help release them from the tray. 

Serve and enjoy! Whip up a batch of hash browns or home fries to go alongside this egg muffins recipe as a great option for weekend breakfasts with the whole family.