Combine the beef with the light soy sauce, Shaoxing wine, cornstarch, and oil. Rest for 20 minutes.
Stir together the sauce ingredients. Gently separate the fresh rice noodles without tearing them.
Sear:
Heat a wok over high heat until just smoking. Add 1 tablespoon oil, spread in half the beef, and sear for about 60 seconds per side. Repeat with another tablespoon oil and the remaining beef; set aside.
Add the final tablespoon oil and the onion. Stir-fry for 1 minute, then add the noodles and toss gently for 2 minutes.
Finish:
Pour the sauce around the hot sides of the wok and toss until the noodles are evenly colored.
Return the beef, add the bean sprouts and scallions, and toss for 1 to 2 minutes until the sprouts soften slightly. Serve immediately.
Notes
Fresh ho fun noodles give the best supple texture; soften refrigerated noodles briefly at room temperature before separating.
Work in batches so the beef sears instead of steaming.