This is a technique not so common in New Zealand and it doesn't really feature in British cookery either (where I am from originally), but I got a taste for it when I lived in Poland and Sweden.
The best know of this technique is of course sauerkraut - soured cabbage, but just as popular in Poland and scandinavia is soured gerkins
The method is similar take vegetable, shred in the case of the cabbage, leave whole in the case of gerkins. Imerse in brine and leave to ferment. Once fermented store airtight.
Cabbage is usually done in a bucket, and you can also sour whole small cabbages (they make a tasty alternative to stuffed cabbage leaves Gołąbki). I have only ever trie white cabbages, I do not know what the result wold be with green or purple. Cover with the brine which should be strong enough to preserve but not strong enough to prevent fermentation or make the cabbage taste like you have fished it out of a salt cellar. Take a plastic bag with no holes in, half fill with water, tie the neck in a knot and place on top of the cabbage, this should force the cabbage to sink and create an airlock. If you have a lid for the bucket put it on, else, place a plate over the top to further keep out bacteria. Leave in moderate room temperature (ie it is usually too warm to do mid summer in NZ) for about two weeks. You can tell by the sour smell that it is done. Do not keep prodding and poking it else everytime yu take off the lid you run the risk of adding in moulds.
Gerkins are done in a similar way, though traditionally you add in a dill flower to the water. In m garden I find that the dill has been and gone by the time the gerkins are ripe for picking, so you can just add fresh leaves or a few seeds. Whole peeled cloves of garlic are also often added. The gerkins are place directly into jars before fermentation and lids placed on them, but not so tight that air cannot escape. Place them in a cool place where the liquid bubbling out of the jars will not be a problem. After a week or two (depending on temperature). Tigten the lids, no need to use a hot water bath. These gerkins will keep up to a year in the fridge or cool store. That is if you can resist the urge to scoff the lot in a week.
I used to take a shopping trip to Gdansk market. There the sellers would have both sauerkraut and soured gerkins (kapusta kiszone and Ogórki kiszone) for sale in huge barrels and you would buy them loose by the kilo. There was nothing more perfect than wandering round the market scoffing them.
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Surströmming
Is another fermented dish I have come accross, however I have never dared make it, but should I miraculously ever catch fish (have treid but always failed), then it would be an interesting experiment.
If you are brave enough there are a couple of things to observe
1. ALWAYS open the tin under water outside
2. ALWAYS eat it outside
The flavour is not unpleasant, rather mild, the texture similar to rollmop herrings, though a little softer.
The smell however is bad! very, very bad! I had not been warned about this and did not follow the two rules. It took two weeks of the windows constantly open and joss sticks going to get the stentch out of my house. The smell probably would not be so bad if the fish is rinse prior to serving.
I recommend eating it with flat bread and soured gerkins, it really did taste rather rice.