Separate and rinse lettuce; pat dry. Cut lettuce crosswise into 1-inch strips; set aside. Cut beef across grain into thin slices. Combine 1 tablespoon stir-fry sauce and garlic in small bowl; stir in beef to coat. Heat 1 tablespoon oil in hot wok or large skillet over high heat. Add beef and stir-fry 1 minute; remove. Heat remaining 1 Tbsp oil in same pan. Add onion and ginger; stir-fry 2 minutes. Add lettuce; stir-fry 2 minutes longer. Add beef, tomatoes and remaining 3 Tbsp stir-fry sauce; cook and stir until vegetables are coated with sauce and tomatoes are just heated through. Serve immediately with peanuts.