Preheat oven to 350°F (175°C) and generously grease a 12-cup muffin tin with non-stick cooking spray.
In a large mixing bowl, whisk together the eggs, milk, salt, black pepper, and garlic powder until well combined and slightly frothy.
Divide the chopped spinach, crumbled feta, and sun-dried tomatoes evenly among the 12 muffin cups.
Pour the egg mixture over the fillings in each cup, filling them to about 3/4 full.
Bake for 20 to 25 minutes or until the eggs are set and the tops are slightly golden.
Remove from the oven and let cool in the tin for 5 minutes before using a knife to gently loosen the edges and lift the muffins out.
Serve immediately or store in an airtight container in the refrigerator for up to 4 days.