In a medium sized, non-stick skillet, heat the oil over medium heat. Add the onion and cook for 2 minutes, stirring once or twice. Add the tomatoes and cook for 1 minute more. Transfer the onion-tomato mixture to a bowl and set aside.
In a medium bowl lightly beat together the whole eggs, egg whites and water, if using. Pour the egg mixture into the skillet and cook over a medium-low flame, stirring frequently, until the eggs are almost set. Drain any excess water from the tomato mixture and stir the mixture gently into the eggs. Toss in the dill, season with salt and pepper, and serve.
Serving suggestion: Serve with a piece of whole-grain toast spread with 1/8 of an avocado, instead of butter.