Gai choy (Brassica juncea), the Chinese leaf mustard or mustard cabbage, is a large, robust, cool-season brassica grown for its broad, cupped, blue-green leaves and their thick, juicy pale ribs. It carries the frank horseradish-and-mustard heat of its species when raw, mellowing to a savoury, faintly bitter green when cooked. Far bigger and more pungent than the tender Western mustard greens of the salad bowl, it is above all a vegetable for pickling and fermenting — the leaf that becomes the salt-cured haam choy of Cantonese kitchens and the sour suan cai of Sichuan — as well as being stir-fried hard with garlic and ginger or dropped into brothy soups.