01 - Combine the sliced flank steak with soy sauce, cornstarch, Shaoxing wine, and oil in a bowl. Mix thoroughly and let marinate for 10–15 minutes to tenderize.
02 - Whisk together light soy sauce, dark soy sauce, oyster sauce, sugar, and water in a small bowl until sugar dissolves. Set aside for quick assembly during cooking.
03 - Gently separate fresh rice noodles. If refrigerated and stiff, steam or microwave briefly for 20–30 seconds to soften without overcooking.
04 - Heat 1 tbsp vegetable oil in a large wok over high heat until smoking. Add marinated beef in a single layer and sear for 1–2 minutes until just browned. Remove immediately and set aside.
05 - Add another 1 tbsp oil to the wok. Stir-fry sliced onions and most of the spring onions, reserving a portion for garnish, for 1–2 minutes until fragrant and slightly softened.
06 - Add rice noodles to the wok, spreading them evenly across the cooking surface. Let sear undisturbed for 1 minute to develop a slight char and enhance flavor.
07 - Pour the prepared sauce over the noodles. Toss quickly and vigorously to coat evenly, allowing the sauce to caramelize slightly on the noodles.
08 - Return the seared beef to the wok along with bean sprouts. Stir-fry for 1–2 minutes until everything is heated through. Garnish with reserved spring onions and serve immediately.