Pickled mustard greens are also known as si chuan pao cai (四川泡菜), or suan cai (酸菜). The leafy vegetable is pickled in a spiced brine which provides a zesty, pungent, and sour taste. Unlike northern style pickled napa cabbage, which is similar to German sauerkraut, the Sichuan pickled mustard greens consist of the entire head of the vegetable and have a slightly sweet flavor. It is used in soups and stews, and goes very nicely with fish. Sichuan suan cai yu (酸菜鱼), or poached fish with pickled mustard greens, is a famous dish that uses this ingredient.
You can find this type of pickle in Chinese and Asian markets. It might go by a different name, such as sour mustard, as shown in the picture above.