01 - Combine the sliced flank steak with soy sauce, cornstarch, Shaoxing wine, and vegetable oil in a bowl. Mix thoroughly and let marinate for 10 to 15 minutes at room temperature.
02 - Whisk together light soy sauce, dark soy sauce, oyster sauce, sugar, and water in a small bowl until the sugar dissolves completely. Set aside for later use.
03 - Gently separate fresh wide rice noodles. If they are stiff from refrigeration, steam briefly or microwave for 30 seconds to soften without overcooking.
04 - Heat 1 tablespoon vegetable oil in a large wok over high heat until smoking. Add the marinated beef in a single layer and sear for 1 to 2 minutes until browned on the edges but still slightly pink in the center. Remove and set aside on a plate.
05 - Add another tablespoon of oil to the wok. Toss in the sliced yellow onion and most of the spring onions, reserving a portion for garnish. Stir-fry for 1 to 2 minutes until the onion begins to soften.
06 - Add the rice noodles to the wok, spreading them out in an even layer. Allow them to sear undisturbed for about 1 minute to develop a light char and enhance flavor.
07 - Pour the prepared sauce over the noodles and quickly toss everything together with tongs or a spatula until the noodles are evenly coated and achieve a rich, glossy appearance.
08 - Return the seared beef to the wok along with the bean sprouts. Stir-fry vigorously for 1 to 2 minutes until all components are heated through and the bean sprouts maintain their crunch.
09 - Transfer to a serving platter and garnish with the reserved spring onions. Serve immediately while hot and fragrant.