**Preheat the oven**
Preheat your oven to 350°F (175°C). Spray a muffin tin with nonstick spray or use silicone muffin liners to prevent sticking.
**Mix the ingredients**
In a mixing bowl, whisk together the egg whites, garlic powder, salt, and black pepper. Stir in the diced bell peppers, spinach, and shredded cheese (if using).
**Fill the muffin tin**
Pour the egg white mixture evenly into the prepared muffin tin, filling each cup about three-quarters full.
**Bake**
Bake in the preheated oven for 18–20 minutes, or until the egg whites are set and slightly golden around the edges.
**Cool and serve**
Let the egg white bites cool for 5 minutes before removing them from the muffin tin. Serve warm or store for later.