To marinate the beef, in a bowl, add the beef and sprinkle in the salt first, mix until absorbed, then add the white pepper, Shaoxing wine, egg white, and cornstarch and mix well, then add the cooking oil to seal and let marinate while preparing the other ingredients.
To prepare the noodles, in a pot over high heat, bring water to a boil, add the chow mein noodles and blanch for about 20 seconds until just softened, then drain, rinse with cold water, and drain well.
To make the sauce, in a bowl, combine the soy sauce, dark soy sauce, oyster sauce, water, and white pepper and mix well.
To stir-fry the beef, in a wok over high heat, heat 2 tablespoons of cooking oil, add the beef and spread it out slightly, let it sear briefly before stir-frying for about 2–3 minutes until just cooked, then remove.
To stir-fry the vegetables, in the same wok over high heat, add 1 tablespoon of cooking oil, add the snap peas, onion, and shiitake mushrooms, and stir-fry for about 1 minute until fragrant, then remove.
To stir-fry the noodles, add the remaining 1 tablespoon of cooking oil, add the noodles and stir-fry for about 1 minute until slightly crisp and fragrant.
To combine, return the beef and vegetables to the wok, pour in the sauce, and toss everything together, then add the mung bean sprouts and scallions and give a quick final toss.
To serve, transfer to a plate and serve hot.