Pickled Herring in Tomato Sauce with Onion

Skin the herring fillets and cut them into strips. If using whole fish, gut and debone it, then cut into bite-sized pieces.

Peel the yellow onion and slice it into rings or half-rings.

In a large saucepan, combine the ketchup, water, apple cider vinegar, Dijon mustard, sunflower oil, salt, sugar, allspice, black pepper, and bay leaves.

Place the saucepan over medium heat and bring the mixture to a boil. Once boiling, reduce the heat and let the marinade simmer for another 2 minutes so the spices fully release their flavor.

Remove the saucepan from the heat and let the marinade cool completely to room temperature.

Layer the sliced herring and onion in a glass container.

Pour the cooled marinade over the fish, making sure all the pieces are evenly coated.