Preheat oven to 350°F (175°C) and grease a 12-cup muffin tin with non-stick cooking spray.
In a large bowl, whisk the eggs, milk, salt, pepper, garlic powder, and oregano until the mixture is uniform and slightly frothy.
Evenly distribute the chopped spinach, crumbled feta, and sun-dried tomatoes into the bottom of each muffin cup.
Pour the egg mixture into each muffin cup, filling them approximately 3/4 of the way to the top.
Bake for 20 to 22 minutes, or until the eggs are set and the tops are lightly golden brown.
Allow the muffins to cool in the pan for 5 minutes to ensure they firm up before removal.
Use a silicone spatula or knife to gently release the muffins from the tin and serve.