Preheat: First, preheat your oven to 375°F (190°C). Generously grease a muffin tin with oil or cooking spray.
Whisk Eggs: Next, in a large bowl or measuring cup, whisk together the eggs, milk, garlic powder, salt, and pepper until fluffy.
Add Fillings: Then, distribute the chopped spinach and crumbled feta cheese evenly into the bottom of the 12 muffin cups.
Pour: Pour the egg mixture over the fillings, filling each cup about 3/4 of the way full.
Bake: After that, bake for 20-25 minutes, or until the muffins are set in the center and slightly golden on top.
Cool: Finally, let them cool in the pan for 5 minutes before removing. Enjoy warm or store for later.