Slice the flank steak thinly across the grain. In a bowl, combine the beef with 1 tbsp light soy sauce, 1 tbsp Shaoxing wine, cornstarch, and baking soda. Mix well and let marinate for 15 minutes.
Gently separate the fresh wide rice noodles into strands; if stiff, warm briefly to loosen. If using dried noodles, soak per package directions until just tender, then drain very well. Rinse and drain the bean sprouts, and cut the Chinese chives into 2-inch lengths.
In a small bowl, stir together the remaining 1 tbsp light soy sauce, dark soy sauce, oyster sauce, sugar, and white pepper until the sugar dissolves.
Heat a wok over high heat until lightly smoking. Add 1.5 tbsp neutral oil and swirl to coat. Add the marinated beef in a single layer and sear undisturbed for 30 seconds, then stir-fry until just browned but still slightly pink, 1–2 minutes total. Transfer to a plate.
Return the wok to high heat and add the remaining 1.5 tbsp neutral oil. Add the rice noodles, spread them out, and let them sear without moving for 45–60 seconds to pick up a little char. Toss gently to separate and heat through, 1–2 minutes.
Push the noodles up the sides of the wok. Pour the sauce mixture into the center and toss to coat evenly. Add the bean sprouts and Chinese chives and stir-fry until the noodles are glossy and tender with a few charred edges and the sprouts are crisp-tender, 1–2 minutes.
Return the beef and any juices to the wok. Splash the remaining 1 tbsp Shaoxing wine around the sides of the wok so it sizzles, then toss everything until the beef is just cooked through and the pan is sizzling and lightly caramelized, 30–60 seconds.
Drizzle with toasted sesame oil, give a final toss, and serve immediately while hot.