Preheat the oven to 375°F (190°C).
Grease a 12-cup muffin pan with cooking spray or olive oil.
Dice the bell pepper, chop the spinach, and slice the green onions.
Whisk the eggs, milk, salt, and black pepper together in a large bowl.
Stir in the bell pepper, spinach, green onions, and cheddar cheese.
Divide the mixture evenly among the prepared muffin cups, filling each about three-quarters full.
Bake for 18–22 minutes, or until the eggs are fully set and lightly golden.
Allow the muffins to cool for a few minutes before carefully removing them from the pan.
Serve warm.