Preheat your oven to 350°F (175°C) and lightly grease a muffin tin with cooking spray or line it with muffin liners.
In a mixing bowl, whisk the egg whites until frothy. Add salt and pepper to taste.
Gently fold in the diced vegetables, shredded cheese, and cooked meat if using. Ensure the ingredients are evenly distributed throughout the egg whites.
Divide the mixture evenly among the muffin cups, filling each about three-quarters full.
Bake in the preheated oven for 20-25 minutes, or until the bites are set and lightly golden on top.
Let the egg white bites cool in the tin for a few minutes before transferring them to a wire rack. Enjoy them warm or store them for later!