Preheat the oven to 350°F (175°C). Grease a muffin tin with cooking spray or oil.

In a mixing bowl, whisk together the eggs and milk until well combined.

Stir in the cheese, spinach, bell pepper, onion, salt, and black pepper.

Pour the egg mixture evenly into the prepared muffin tin, filling each cup about 3/4 full.

Bake in the preheated oven for 20-25 minutes, or until the egg muffins are set and lightly golden on top.

Remove from the oven and let cool for a few minutes before removing from the muffin tin.

Serve warm or store in the refrigerator for later use.