Preheat your oven to 350°F (175°C). Generously grease a 12-cup muffin tin with cooking spray or butter.
In a large bowl, whisk together eggs, milk, salt, pepper, and garlic powder until well combined and slightly frothy.
Distribute the cooked sausage, bell pepper, spinach, and green onions evenly among the 12 muffin cups.
Pour the egg mixture over the vegetables and sausage, filling each cup about ¾ full.
Sprinkle shredded cheddar cheese on top of each muffin.
Bake for 20-22 minutes, until the eggs are set and the tops are lightly golden. A toothpick inserted in the center should come out clean.
Let cool in the pan for 5 minutes, then run a knife around the edges to release. Store in an airtight container in the refrigerator for up to 5 days. Reheat individual muffins in the microwave for 30-45 seconds.