Salt the mackerel fillets on both sides, place them in a tray and refrigerate for 1 hour. Quickly rinse the mackerel fillets in a bowl of water (the flesh of the fish is delicate, so don’t wash it under running water). Pat the mackerel dry with paper towel.
Place the mackerel fillets in a deep tray and top with the kombu. Pour in enough vinegar so that the fish is just covered. Marinate