Whisk together the egg whites, non-fat milk, salt, and pepper in a medium bowl until frothy.
Heat the olive oil in a non-stick skillet over medium heat.
Add the diced bell peppers and onions to the skillet, sautéing until soft and slightly caramelized.
Pour the egg white mixture into the skillet, covering the vegetables evenly.
Allow the egg whites to cook undisturbed for a couple of minutes until the edges begin to set.
Gently lift the edges of the omelet with a spatula, tilting the pan to allow the uncooked egg whites to flow to the edges.
Once the omelet is almost set but still slightly runny on top, sprinkle the chopped spinach and shredded cheese (if using) over half of the omelet.
Carefully fold the omelet in half, covering the fillings.
Cook for another minute or until the cheese is melted and the omelet is fully set.
Toast the whole-grain bread to your desired level of crispiness.
Prep: 10 minutes
Cook: 10 minutes
Total Time: 20 minutes
Servings: 1Cooking Time: 10 minutesTotal Time: 20 minutesKcal: Approximately 300 kcal for the omelet and toast (varies based on bread and cheese choice)