To marinate the beef, in a bowl, mix the beef with the salt until fully absorbed, then add the white pepper, Shaoxing wine, egg white, cornstarch mixed with water, and cooking oil and mix well, then let marinate while preparing the remaining ingredients.
To prepare the noodles, place the hor fun noodles on a plate and microwave for 2–3 minutes until softened, then gently separate into loose strands.
To make the sauce, in a bowl, mix the soy sauce, dark soy sauce, oyster sauce, water, and white pepper.
To stir-fry the beef, in a wok over high heat, add 2 tablespoons of cooking oil and cook the beef for about 2–3 minutes until just browned, then remove.
To stir-fry the onion, in the same wok over high heat, add 1 tablespoon of cooking oil and cook the onion for about 30 seconds until fragrant, then remove.
To heat up the noodles, in the wok over high heat, add the remaining cooking oil and the noodles and gently toss until heated through.
To combine everything, then return the beef and onion, add the sauce, and toss lightly to combine, then add the bean sprouts and scallions and toss briefly before serving.