Preheat your oven to 350°. Prepare your muffin tin by spraying with nonstick spray. Set aside.
In a large bowl, whisk together the egg whites, bell peppers, onion, spinach, scallions, salt, and pepper. Mix everything until well combined.
Using a ⅓ measuring cup, scoop the mixture evenly into the muffin cavities. You should have enough to make 12 muffins.
Bake for 20-22 minutes or until muffins have puffed up and egg whites are completely cooked (not wet/jiggly in the middle). The muffins will sink back down as they cool.
Muffins are best served warm. They can be stored in the fridge for up to 5 days and reheated in the microwave.