Preheat oven to 350 degrees Fahrenheit. In a bowl, whisk egg whites by hand or with a hand mixer. Set aside.

In a skillet, heat oil and sauté onions and peppers. Add to the egg whites.

Add spinach, cheese, salt, and pepper, mixing to distribute evenly.

Spray 12 muffin cups with nonstick cooking oil spray or line a muffin pan with liners. Distribute mixture evenly between the muffin cups.

Bake 15–20 minutes. Serve warm or store in the fridge until ready to eat.