Preheat your oven to 350 degrees.
Spray a large skillet with cooking spray and cook the onions until they are translucent. Add the spinach and cook until wilted.
In a large bowl, whisk the eggs. Then stir in the artichoke hearts, mozzarella cheese, feta, salt, garlic powder, and pepper. Then stir in the onion spinach mixture.
Spray a muffin tin heavily with cooking spray. Pour a scant 1/3 cup of the mixture into each hole of the muffin tin and bake for 20 to 25 minutes or until a toothpick inserted into the center of the muffin comes out clean.
To freeze: Allow muffins to cool completely. Set on wax paper in a single layer and freeze for 20 minutes. Then transfer to an airtight container (stacking if necessary) and return to the freezer. To reheat, microwave for 30 to 60 seconds.