Preheat the oven to 350°F (175°C) and grease a muffin tin with cooking spray or olive oil.
In a large mixing bowl, whisk together the eggs and milk until well combined.
Stir in the diced bell peppers, chopped spinach, onion, shredded cheese, salt, and pepper. Mix until all ingredients are evenly distributed.
Pour the egg mixture evenly into the prepared muffin tin, filling each cup about 3/4 full.
Bake in the preheated oven for 18-20 minutes, or until the frittatas are set and lightly golden on top.
Allow to cool for a few minutes before carefully removing the frittatas from the muffin tin.