Preheat your oven to 350°F (175°C).
Whisk the egg whites in a bowl until frothy.
Season with salt and pepper.
Add in the shredded cheese, diced bell peppers, and spinach.
Lightly spray a muffin tin with olive oil.
Pour the egg white mixture evenly into each muffin cup.
Bake for 15 minutes or until set and lightly golden.
Let cool for a few minutes before removing from the tin.
Top with optional ingredients like diced tomatoes or herbs.
Serve warm or store for later.