Cut tomatoes into wedges (makes about 2 cups); set aside. Cut liver into 1/2" wide strips; set aside. In a medium bowl, combine soy sauce, sherry, cornstarch, sugar, salt and ginger. Add liver; marinade for 15 to 20 minutes. In a medium skillet or wok, heat 2 tablespoons of the water. Add liver and marinade to skillet. Cook and stir until partially cooked, about 2 minutes. Remove from skillet and set aside. To the skillet add remaining 2 tablespoons water. Add onions; cook and stir until onions are crisp and tender, about 3 minutes. Add tomatoes; cook and stir until tomatoes are tender, 1 minute. Add reserved liver and marinade; cook and stir until meat is cooked, but still pink in the center, about 2 minutes Serve with cooked rice, if desired.