Preheat the oven to 350°F (175°C).
Generously grease a standard 12-cup muffin pan with cooking spray or softened butter.
Finely dice the bell pepper, chop the spinach, and slice the green onions.
Crack the eggs into a large mixing bowl.
Add the milk, garlic powder, salt, and black pepper and whisk until completely combined.
Stir in the bell pepper, spinach, green onions, and about three-quarters of the shredded cheddar cheese.
Divide the egg mixture evenly between the prepared muffin cups, filling each approximately three-quarters full.
Sprinkle the remaining cheddar over the tops.
Bake for 18–22 minutes, until the centers are set and the edges are lightly golden.
Allow the egg muffins to cool in the pan for approximately 5 minutes.
Carefully remove from the pan and serve warm, or cool completely before storing for meal prep.