In a small bowl, whisk eggs, water, salt and pepper until blended. In a large nonstick skillet, heat butter over medium-high heat. Pour in egg mixture. Mixture should set immediately at edge. As eggs set, push cooked portions toward the centre, letting uncooked eggs flow underneath.
When eggs are thickened and no liquid egg remains, add cheese, tomato and green onion to 1 side. Fold omelet in half and cut into 2 portions; slide onto plates.