- Cut beef across grain into thin slices 1-inch wide; coat with 1 Tbsp. stir-fry sauce. Let stand 30 minutes.
- Meanwhile, cook noodles according to package directions in lightly salted water; drain.
- Combine remaining 6 Tbsp. stir-fry sauce with beef broth and pepper.
- Heat 1 Tbsp. oil in hot wok or large skillet. Add beef and stir-fry 1 minute; remove.
- Heat remaining oil in same pan. Add garlic and ginger; stir-fry 10 seconds, or until fragrant. Add onion; stir-fry 2 minutes. Add romaine; stir-fry 2 minutes longer.
- Reduce heat to medium. Add noodles and stir-fry sauce mixture; cook, stirring, until noodles are thoroughly heated, about 2 minutes.
- Stir in beef; heat through. Serve immediately.