Recipe:

Brine sardines in a 10% salt solution for 48 hours. Shorter than this and the fish turn out mushy, longer and they're too salty.

Rinse brine off, put fish in a jar with ~65% vinegar solution and whatever spices you like. I use bay, onion, pepper and junpier.

Wait 4 days before eating. Obviously it needs to be kept in the fridge.