2. In a small bowl, whisk the egg whites together with salt and pepper.
3. Spray the same skillet with nonstick spray and heat it over medium-high heat. Pour in half of the egg white mixture and immediately stir with a spatula. While stirring, quickly shake the skillet in a back and forth motion directly on the heat surface. (Do not lift the pan off the heat surface, as this will cool the pan.) This step should not take more than 15 seconds.
4. When the egg whites are almost set, remove the pan from the heat and add half of the cheddar cheese, cooked spinach and sliced tomatoes across the center the omelet.
5. Fold the omelet in half and turn it onto a warm plate. Wipe out the pan with a paper towel and repeat with the remaining ingredients