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.
Herring and sardine are basically the same fish but one prefers cool, other warm, and they share waters between. My local market I can get either as whole fresh fish for about 6.50€/kg. Worth checking out as you likely have one of them depending where you live in the world.
I actually just get them for my dogs because they’re abundant, cheap, yummy, and with high protein and omega-3.