Pour the soy sauce, oyster sauce, pepper and cornstarch over the beef, and toss to coat. Marinate at room temperature for 15 minutes
Gently separate the flat rice noodles so they are not clumpy. Drizzle some peanut oil over the noodles to loosen them if necessary
In a small bowl, prepare the sauce by mixing together the soy sauce, dark soy sauce, shaoxing wine, brown sugar, and white pepper. Set it aside.
In a wok over medium-high heat, heat 1 tbsp of peanut oil. Add the beef and stop-fry until just no longer pink. Remove it and set aside.
Add the remaining 1 tbsp of oil to the wok, then toss in the garlic and onion. Stir-fry just until the onion becomes translucent
Increase the heat to high. Add the flat rice noodles to the wok, separating them, then add the sauce. Stir-fry until all the noodles are evenly coated.
Return the beef to the wok. Toss in the bean sprouts, scallions and bok choy at the last minute and turn off the heat. Serve immediately.