Pickled mustard greens are mustard greens that have been brined in a salt solution and then fermented for a few days to develop their salty-sour flavor. It is used in Asian cooking and is most commonly used in South East Asian dishes.

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How To Use It
Pickled mustard greens are pretty versatile and can be used in many different ways.
It can be steamed or boiled with fish and meats, added to soups, or chopped into small pieces and eaten as is as a snack with a side of peanuts or used as an accompaniment alongside other dishes - almost like a palate cleanser.
Pickled mustard greens have a more salty flavor than sour but this is dependent on how long it was allowed to ferment. To me, it tastes almost like sauerkraut, so if you can envision how sauerkraut tastes like, this is pretty much on par with it.
The leaves of the pickled mustard greens are soft but the stems of the vegetables maintain a bit of a crunch still since mustard greens stems are usually thick in width so they can hold it's shape and texture quite well, even after pickling them.
How to Substitute Pickled Mustard Greens
A good substitution for pickled mustard greens would be sauerkraut.
Where Do I Buy It?
This is most commonly found in Asian grocery stores and packaged in a vacuum packed plastic packaging. I personally have not seen any in glass containers, but if I ever do I'll update you with the details.
There are a gazillion different pickles and preserved vegetables in the Asian supermarkets so it may be intimidating to find it.
To find the right one, make sure you see the word "Mustard Green" in the ingredients list. It can also be called Suan Cai, or 酸菜. You can use the Chinese wording here to compare it to the labels at the store.
You might come across some that say "Mustard Stems" that have chili in the ingredient list, which is a totally different type of preserved vegetable and not the same thing as Pickled Mustard Green.
How to Store It
If you make this from scratch, you can treat this like kimchi or any type of lacto-fermented pickle and store it in the fridge for up to 6 months as long as the pickles are fully immersed under the brine.
If you buy the pre-packaged pickled mustard greens (they are most commonly packaged in sealed plastic packaging), I wouldn't keep it for more than a week or so because they usually don't come with enough brine to fully immerse the pickle in once you open the package so it is exposed to oxygen - which can speed up bacteria growth.
Recipes That Use Pickled Mustard Greens
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heather (delicious not gorgeous) says
i saw pickled mustard greens and my mind immediately went to KAU YUK. there's a lot of good japanese and korean and vietnamese food around me, but not much chinese so i either need 1) learn how to make kau yuk or 2) go home. i'm thinking i'll go with the latter (;
Joyce says
Hahaha I know exactly what you mean. Although I think kau yuk uses a different pickled veggie? I believe it uses mui choy but gosh now I'm craving it haha!