They are not like American mustard greens at all.
Preserved Mustard Greens are used as an ingredient in dishes, or they can be served as an appetizer. They are often eaten with congee for breakfast.
In northern China, Napa cabbage is used; in the south, they use the mustard plant called “jìe cai” (“gai choy”) in Cantonese. This plant has a slightly bitter taste with a mild hint of horseradish to it.
The pickling process used to preserve the cabbage is actually a fermentation process, as is used in making Sauerkraut. The process sweetens the mustard plant leaves a bit. Whether cabbage or mustard plant leaves are used, they end up with a fair bit of their crunch left, and having a salty, sour taste.
Usually only stems and young leaves are pickled; older dark green leaves are peeled off. Otherwise, the cabbages (or mustard plants) being used are pickled whole. They are dipped into boiling water (usually), then put in a pottery jar with a brine of cold water and salt, and then a very heavy weight is placed on top so that the greens get gradually pressed down as they ferment.
Preserved Mustard Greens can be bought in vacu-paks.
Rinse in cold water before using. They will be quite salty, even when rinsed, so resist temptation to overly salt a dish.
Chop into 1 inch pieces and use as a topping for dishes, or add to dishes at the end of cooking.
To stir fry Preserved Mustard Green on its own, soak in water, then rinse, and stir fry in a stainless steel wok with vinegar and sugar (stainless steel won’t react with the vinegar.)
Store in unopened package in refrigerator for up to a few months.
Preserved Mustard Greens are called “cai chua” in Vietnamese.