Cut beef into 2x1x1/4" slices. In a small bowl, toss meat with cornstarch and soy sauce to coat evenly. Cut tomatoes into 6 wedges each. In a wok, heat oil over high heat until smoking. Add scallion and ginger and stir fry until aromatic, about 30 seconds. Add onion and stir fry until onion is soft, about 2 minutes. Add beef and stir fry until meat is just browned, about 3 minutes. Add tomatoes and 1 Tblsp. water; toss to mix and heat through, 30-60 seconds. Remove from heat. Add sesame oil, stir to blend, and serve.