In a medium nonstick skillet, heat the oil over medium heat.
Chop onion and add to hot skillet; cook for 2 minutes, stirring once or twice.
Chop tomato and add to onion, cook for 1 minute more. Transfer the onion-tomato mixture to a bowl and set aside.
In a medium bowl, lightly beat together the eggs, egg whites, and water (optional). Pour the egg mixture into the skillet and cook over medium-low heat until they are almost set; stir frequently to scramble.
Drain any excess water from the tomato mixture, then stir gently into the eggs. Flavor with dill, salt, and pepper to taste.