Preheat your oven to 350°F (175°C) and lightly grease a muffin tin.
In a mixing bowl, whisk together the egg whites until slightly frothy.
Stir in spinach, bell pepper, onion, and cheese if using.
Season with salt and pepper, then pour the mixture evenly into the muffin cups.
Bake for 18 to 20 minutes, or until the egg bites are set and slightly golden on top.
Remove from oven, let them cool for a few minutes, then pop them out of the muffin tin and enjoy.