This is a salt fermented pickled vegetable made from the lower stems of a mustard cultivar with strangely swollen stems. This variety appeared in Sichuan provence and first became popular there, but it is now also used in other cuisines of Southern China. It is very salty and needs to be rinsed thoroughly before use. The photo specimens, purchased out of a tub in a Los Angeles Asian market, were about 3 inches in diameter and weighed 8-3/4 ounces each. Photo © cg1.
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Buying: Some of the larger Asian markets in Los Angeles occasionally have this in bulk tubs, but generally you will find it put up in 14 ounce red and yellow cans. Around here they are White Rabbit brand, but others report My Ling brand. The cans may contain whole stems or shredded, check the label.
Storage: Fresh or from a can, it'll keep a couple weeks in a sealed container in the refrigerator. Unopened cans will keep more than a year.
Prep: Slice and/or cut to the form you wish to use, then rinse it thoroughly in warm water to reduce the salt somewhat. It'll still be pretty salty.
Cooking: This vegetable is used both raw and cooked in various recipes, but most commonly in soups. It is used sparingly because of its saltiness and distinctive flavor.