Marinate the Beef: In a bowl, toss sliced beef with 1 Tbsp soy sauce, Shaoxing wine, cornstarch, and ½ tsp oil. Let rest 10–30 minutes.
Blanch Broccoli: Boil salted water, cook broccoli 1 minute until bright green, transfer to ice water, then drain.
Cook Noodles: Prepare noodles per package until al dente; drain and toss with 1 tsp sesame oil.
Stir‑Fry Beef: Heat 1 Tbsp vegetable oil in a wok over high heat. Cook beef in batches 1–2 minutes until just browned. Remove.
Aromatics: Add remaining oil, then onion (if using), garlic, and ginger. Stir 30 seconds.
Combine & Sauce: Return beef and broccoli to the wok. Whisk broth, soy sauce, oyster sauce, hoisin, sugar, white pepper, and a splash of cornstarch slurry; pour in and cook 1–2 minutes until thickened.
Toss with Noodles: Add noodles; gently fold until everything is evenly coated and heated through.
Finish & Serve: Drizzle with sesame oil, top with green onions and sesame seeds, and add chili oil if desired.