Make the brine by dissolving the salt in water. If we aren't bothered by the smell and have enough space, simply covering the fish slices in salt would do as well.
Gut the fish, remove the heads and scales. With a bit of luck we can ask someone to do this for us where we buy the fish.
Boning is not necessary if we aren't bothered about the spine which is a bit crunchy. Some like it others don't. We can take it out by simply grabbing the spin at the exposed end and pulling it out. Sometimes, we need to open the fish first simply by prying it open with our fingers.
Rinse the pieces of fish well then place them into the brine in the fridge or under the brine in case of using dry salt.
After about a day take them out then rinse them off the salt. The flesh need to be firm to the touch now as if it's cooked dry. Otherwise it needs to go back under the brine for a day more.
Fill the middle of the jar up with the onion slices. Add garlic, black pepper, mustard seed, salt and any other optional ingredients we wish for.
Place the fish slices in, fill it up with half vinegar half water mixture. Put the lid on then place the jar into the fridge for another day. In case the liquid gets very cloudy that means the brining and rinsing process wasn’t really successful. Eat it up in a couple of days until it’s still edible. If the brining process was successful we can store our pickled herring or sardines in the fridge for up to a month easily. Maybe even more but who can wait that long.