In a refrigerator container, combine the tomatoes, cucumber, and scallions. Add the dressing, and season to taste with salt and pepper; mix well. Cover and refrigerate for 4 hours or overnight, stirring the salad several times.
To serve, drain off most of the dressing. Place a lettuce leaf on each of 6 salad plates and spoon salad into each leaf. Sprinkle 1 tablespoon Parmesan over each.