01 - Combine the sliced flank steak with soy sauce, cornstarch, Shaoxing wine, and vegetable oil in a bowl. Mix thoroughly to coat each piece 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 completely. Set aside for quick addition during stir-frying.
03 - Gently separate fresh rice noodles. If refrigerated and stiff, steam or microwave for 30–60 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 browned but still pink inside. Remove beef and set aside.
05 - Add another 1 tbsp oil to the wok. Stir-fry yellow onion slices and most of the spring onions, reserving some green portions for garnish. Cook for 1–2 minutes until fragrant and slightly softened.
06 - Add rice noodles to the wok, spreading them in an even layer. Allow to sear undisturbed for 1 minute to develop a slight char and enhance flavor.
07 - Pour the prepared sauce over the noodles and toss quickly and continuously to coat evenly. The sauce should bubble and thicken slightly, coating each noodle strand.
08 - Return the seared beef to the wok along with bean sprouts. Stir-fry for 1–2 minutes until everything is heated through and bean sprouts retain some crunch.
09 - Transfer to a serving platter and garnish with reserved spring onions. Serve immediately while hot and steaming.