Partially freeze steak to firm and slice diagonally across the grain into very thin strips. Combine cornstarch, sugar, and ginger and stir in soy sauce. Pour mixture over meat and stir. Cut green peppers into 1-inch squares and cut tomatoes into wedges. Quickly brown beef strips (1/3 at a time) in hot oil and remove from pan. Reduce heat; add green pepper, garlic and water to pan and cook until green pepper is tender-crisp, 5 to 6 minutes. Stir in meat and tomatoes and heat through.