Cut the beef, across it grains, into thin slices or strips, then place it in a bowl. Add cornstarch, Shaoxing rice wine, and 1 tablespoon of water. Rub well until the liquid is fully absorbed. Add ½ tablespoon of oil and mix to coat the meat evenly.
Mix the sauce
In a small bowl, mix Sha Cha sauce, light soy sauce, dark soy sauce with 3 tablespoon of water.
Cook the noodles (skip if using precooked noodles)
Bring plenty of water to a rolling boil, then add the noodles. Cook until they are just underdone (bite one strand to test). Drain, then cut the noodles into shorter strands using scissors. Set aside.
Sear the beef
Heat an empty wok over high heat until it starts to smoke. Add 1 tablespoon of oil and swirl it around to coat a larger area (If using non-stick cookware, add the oil first, then heat it up.) Add the marinated beef, spreading the pieces out so they sear quickly. Once the bottom side of the meat loses its pink color, flip and toss to sear the other side. Transfer to a plate as soon as no pinkness remains.
Fry the vegetables
Pour the remaining oil into the wok, then add the garlic and onion. Stir-fry until the onion just begins to wilt. Add the chili pepper and bok choy, and stir-fry for about 30 seconds.
Combine the dish
Add the cooked noodles and beef, then toss to loosen the noodles and combine everything evenly. Finally, pour in the sauce mixture and continue tossing until the seasoning is evenly distributed.
Notes
1. Choose beef cuts that are suitable for quick stir-frying, such as sirloin, flat iron, flank, skirt, and tenderloin.2. Learn more about Sha Cha Sauce in the "Ingredient Notes" section above. You may use XO sauce as a substitute.