Preheat your oven to 350°F (175°C).
In a mixing bowl, whisk together the egg whites until frothy.
Add diced bell peppers, spinach, cheese, salt, and pepper to the egg whites.
Grease a muffin tin with cooking spray or line with silicone liners.
Pour the egg white mixture into each muffin cup, filling them about 3/4 full.
Bake in the preheated oven for 15-18 minutes or until the bites are set and lightly golden.
Remove from the oven and let them cool for a few minutes.
Carefully pop the bites out of the muffin tin.
Serve warm or allow to cool completely before storing.
Enjoy as a snack or as part of your breakfast!