Preheat your oven to 375°F and lightly grease three slots of a standard muffin tin with avocado oil.
Season the chicken breast with garlic powder, sea salt, and black pepper.
Grill the chicken over medium-high heat for 5-6 minutes per side until it reaches an internal temperature of 165°F and shows golden char marks.
Let the chicken rest for 5 minutes, then dice it into small, uniform cubes.
Finely dice the red bell pepper and roughly chop the baby spinach.
In a mixing bowl, whisk the eggs until the yolks and whites are fully incorporated.
Fold the diced chicken, bell pepper, and spinach into the egg mixture.
Divide the mixture evenly between the prepared muffin cups and sprinkle the crumbled feta cheese on top.
Bake for 18-20 minutes until the eggs are set and the tops are slightly golden and firm to the touch.
Allow the muffins to cool for 2 minutes before removing them from the tin to serve.