Marinate beef with soy sauce, oyster sauce, cornstarch, and sesame oil. Rest for 15 minutes.
Cook noodles, drain, then pan-fry in oil until crispy on both sides. Set aside.
Stir-fry beef in a hot wok until browned. Remove.
Stir-fry onion, bell pepper, carrot, and garlic for 2–3 minutes. Add bean sprouts and half the green onions.
Mix sauce ingredients, add to wok, and thicken with cornstarch slurry.
Return beef to wok, toss everything together.
Serve over crispy noodles and garnish with remaining green onions.