Preheat the oven to 350°F (175°C) and liberally grease a standard 12-cup muffin tin with cooking spray.
In a large mixing bowl, whisk together the eggs, milk, salt, and black pepper until the mixture is uniform.
Stir the chopped spinach and diced bell peppers into the egg mixture.
Pour the mixture into the prepared muffin tin, filling each cup approximately three-quarters full.
Distribute the shredded cheddar cheese evenly over the top of each muffin cup.
Bake for 20 to 22 minutes, or until the eggs are set and the centers no longer jiggle.
Remove from the oven and allow to cool in the pan for 5 minutes before using a knife to loosen the edges and serve.