1.Sprinkle salt on skin of fish. Heat 1 tablespoon of the oil in wok or large frying pan; cook fish, skin-side down, in batches, until browned both sides and cooked as desired. Cover to keep warm.
2.Heat same wok; stir-fry bok choy and choy sum until just wilted. Add mushrooms, crab and another tablespoon of the oil; stir-fry 1 minute. Add remaining oil with mirin, sauce and stock; stir-fry until heated through.
3.Divide vegetables among serving plates; top with fish, drizzle with sauce.