Soak herring fillets for a few hours (or over the night) in cold water or milk. It will make it more tender and delicate.
Remove the liquids and cut herrings into 2-cm pieces.
Heat the oil in a medium pan.
Fry the onions until translucent.
Add sugar, bay leaves and allspice berries.
Simmer on a small heat for a minute.
Add a pinch of salt, tomato paste, vinegar, raisins, pinch of ginger and pinch of turmeric.
Simmer for 3-4 minutes, or until the onion will be soft. Stir it from time to time.
Leave it to cool.
In a big jar arrange a layer of herrings, cover it with the sauce. Repeat until you run out of ingredients. The last layer should be the sauce.
Leave it in a fridge for at least 3 days for flavours to develop.