In Asia, Mustard Greens are used in many ways, very often as fresh vegetables (small Gai Choy an several others) or pickled (Large Gai Choy).
[Suan Cai (sour vegetable); Dua Cai Chua (Viet); Pak Kad Dong (Thai);
Brassica juncea]
Used in many Chinese soups and stir fries, and also very popular in Vietnam, this is basically sauerkraut made with mustard greens. They are clearly not the same mustard plant that's well known to our Southern cuisine, but an Asian mustard called Large Gai Choy. It has short, heavily ribbed leaves with a distinctive curl to them. While the photo shows a whole large head, Sour Mustard Greens are often made from much smaller plants, and smaller packages may contain only slices of the leaf stems.
Large Gai Choy is also grown in California and sold fresh in the Asian markets here in Los Angeles. Unlike Small Gai Choy, expected to be cooked as fresh greens, the thicker, stronger tasting Large Gai Choy is most often pickled or otherwise processed at home.
Commercially pickled Gai Choy is always imported from Asia, as it is
cheaper to make there, and easily transported. After harvest, the plants,
minus roots, are naturally salt fermented or acid pickled. The photo
specimen, about 13 inches long if fully stretched out, was grown and
processed in Thailand, then vacuum packed in plastic. Ingredients:
Green Mustard, Water, Acetic Acid, Citric Acid Salt,
Monosodium Glutamate (0.10%), FDIC Yellow #5 and #6,
Sodium Metabisulphite.
[Moi Choy, Mui Choy; Brassica juncea]
These preserved greens are not pickled, just dried with a whole
lot of salt. They are quite popular, particularly to cook with pork
belly. To prepare, they need to be rinsed well in several changes
of water to reduce salt content. Because they go directly from field
to salting, they usually need to be float washed to remove sand, as
you would with Spinach.
More on Cabbage Pickles.