1, If using dried noodles, bring a large pot of water to a boil. When the water is boiling, add the noodles and cook until just barely tender (the noodles will finish cooking in the wok, so you do not want to overcook them here). Drain, rinse with cold water, and transfer to a rimmed baking sheet. Toss with a small amount of oil and let stand 15 minutes to dry slightly. If using fresh noodles, they do not need to be boiled; they can be stir-fried directly from the package.
2, In a medium bowl, combine the beef with 1 teaspoon oil, 1 teaspoon fish sauce, 1 teaspoon soy sauce, the cornstarch, and the salt and mix well to coat. Let stand 10 minutes.
3, Heat a wok over high heat; the metal will have a matte appearance and a drop or two of water flicked onto its surface should evaporate on contact. Add 1 tablespoon of the remaining oil and heat until shimmering but not smoking. Add the beef and stir-fry, tossing the meat in the pan, until lightly browned but still pink inside, 5 minutes. Transfer the meat to a plate, rinse the wok with water and return to high heat.
4, Using your fingers, separate the noodles if they are sticking together. Add remaining 2 tablespoons oil to the hot wok. When hot, add the noodles and bok choy. Stir-fry 30 seconds, tossing the noodles in the wok so that they don’t stick. Add the bean sprouts and remaining fish and soy sauces and cook 30 seconds longer, tossing to incorporate the ingredients. Return the cooked beef to the wok and toss with the noodles until everything is heated through and well incorporated.
5, Transfer to a warmed serving platter and serve.