01 - Combine the sliced flank steak with soy sauce, cornstarch, Shaoxing wine, and oil in a bowl. Mix thoroughly until the beef is evenly coated. Let marinate for 10–15 minutes at room temperature to tenderize the meat.
02 - Whisk together light soy sauce, dark soy sauce, oyster sauce, sugar, and water in a small bowl until sugar dissolves completely. Set aside near your cooking station for easy access during stir-frying.
03 - Gently separate fresh rice noodles by hand to prevent breakage. If refrigerated noodles feel stiff or stick together, steam briefly for 1–2 minutes or microwave in 20-second intervals until pliable. Avoid over-softening.
04 - Heat 1 tbsp vegetable oil in a large wok over high heat until smoking. Add the marinated beef in a single layer, spreading pieces apart. Sear for 1–2 minutes until browned but still slightly pink inside. Remove immediately and set aside on a plate.
05 - Add another 1 tbsp oil to the wok. Toss in the sliced yellow onion and most of the spring onions, reserving a portion for garnish. Stir-fry over high heat for 1–2 minutes until the onion becomes fragrant and slightly translucent.
06 - Add the rice noodles to the wok, spreading them across the cooking surface. Let them sear undisturbed for 1 minute to develop a light char and enhance flavor. Do not stir constantly—allow direct contact with the hot wok.
07 - Pour the prepared sauce over the noodles. Quickly toss and fold the noodles using tongs or a spatula, ensuring every strand is evenly coated. Continue stir-frying for 30–60 seconds until the sauce absorbs and the noodles glisten.
08 - Return the seared beef to the wok along with the bean sprouts. Stir-fry everything together vigorously for 1–2 minutes until heated through and the bean sprouts remain crisp-tender. Transfer to a serving platter, garnish with reserved spring onions, and serve immediately while piping hot.