Preheat oven to 350F.
Heat a skillet on medium-low heat. Drizzle with avocado oil. Saute onions until caramelized.
In a large bowl, combine onions, eggs, spinach, feta, salt, and pepper. Mix until completely combined.
Grease a muffin tin. Pour batter into muffin tray. Optional, sprinkle top of individual muffins with shredded cheese.
Bake for 20 minutes or until centers are baked through. Enjoy!