Preheat oven to 350°F. Coat a 12-cup muffin tin with cooking spray.
Whisk eggs, milk, salt and pepper together in a large bowl until well combined and frothy. Fold in cheese, spinach and roasted red peppers until combined.
Divide the mixture among the prepared muffin cups (about ⅓ cup each).
Bake until puffed, set and light golden, 16 to 18 minutes. (The egg bites will deflate once removed from the oven.) Let cool in the pan for 5 minutes before serving.
