Place sauce and stock in a small jug. Whisk to combine.
Heat half the oil in a hot wok. Add beef in two batches. Stir-fry until brown. Remove from wok.
Heat remaining oil in same hot wok. Add onion and capsicum. Stir-fry until onion is almost tender. Add corn and snow peas. Stir-fry until they are tender.
Return beef to wok. Add sauce mixture. Stir-fry until beef is hot.