Thinly slice the Beef Flank Steak across the grain into bite-size strips. Toss the slices with a pinch of Baking Soda and a tablespoon of cold Water, let sit 10–15 minutes to tenderize, then rinse briefly and pat dry.
In a bowl, combine 1½ tbsp Light Soy Sauce, 1 tbsp Shaoxing Wine, 1 tbsp Cornstarch, 1 tsp Sugar, ½ tsp Sesame Oil and a pinch of Ground White Pepper. Add the beef and marinate while you prep the rest (10–15 minutes).
Separate and loosen the Wide Rice Noodles; if refrigerated or stuck together, gently warm them covered with a damp towel or steam for a minute so they become pliable but not mushy.
Heat a wok or large skillet over high heat until very hot, then add 1–2 tbsp Neutral Oil. Stir-fry the marinated beef in batches just until browned and slightly undercooked (about 30–60 seconds per side), then remove and set aside.
Add another tablespoon of Neutral Oil to the wok, toss in the noodles and stir-fry quickly to coat and loosen. Splash in 1 tbsp Light Soy Sauce, 1 tsp Dark Soy Sauce for color, 1 tbsp Oyster Sauce, and a teaspoon of Sugar; toss to combine and heat through. If the mixture is dry, add a tablespoon of Water to help the sauce come together.
Return the beef to the wok and toss everything together over high heat so the flavors meld and the beef finishes cooking. Add the Bean Sprouts and most of the Scallions, tossing just until the sprouts are warmed but still crunchy.
Finish with a drizzle of Sesame Oil and a grind of Ground White Pepper to taste, adjust seasoning with extra soy or a pinch of sugar if needed.
Serve immediately, garnishing with the remaining Scallions for a fresh, crisp contrast to the savory noodles and beef.
