In a medium bowl, whisk together egg whites and water. Season with salt and pepper.
In a nonstick skillet, heat oil over medium-high heat. Add spinach and season with salt and pepper. Stir for a minute, until leaves are wilted and tender.
Add in egg whites and cook until almost set. Using a spatula, lightly lift up sides around the outside of the omelet and let any uncooked egg whites run underneath.
Spread cottage cheese on top of the omelette in the middle, and sprinkle with parmesan. Fold each side of the omelette over onto itself, covering the cottage cheese.
Serve with some diced tomatoes, if desired. Enjoy!