Place noodles in large heatproof bowl, cover with boiling water, stir gently to separate noodles, then drain
Heat half of the oil in wok or large pan, stir-fry beef in batches, until browned and cooked through.
Heat remaining oil in same wok, stir-fry all vegetables including onion until soft add garlic and all other. Return beef to wok with bok choy, combined sauce and noodles, stir-fry until bok choy is just wilted but not too saucy.