- Cut beef across grain into thin slices, then into 1-inch squares. Combine 2 Tbsp. stir-fry sauce and garlic in small bowl; stir in beef. Let stand 15 minutes.
- Meanwhile, combine remaining 3 tablespoons stir-fry sauce and black pepper.
- Heat 1 tablespoon oil in hot wok or large skillet over high heat. Add beef and stir-fry 1 minute; remove.
- Heat remaining 2 tablespoons oil in same pan. Add bell peppers and onion; stir-fry 3 minutes longer. Add beef and stir-fry sauce mixture; cook and stir only until beef and vegetables are coated with sauce and heated through.