The central city of Kyoto is located in an inland area. Before transportation systems were developed, it was hard to get fresh fish, such as mackerel. This sushi was born in the days when mackerel needed to be preserved in salt for shipping.
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 for 20 minutes, then flip the fillets over and marinate for a further 20 minutes (you can marinate the fish for up to 3 hours, depending on how pickled you want it to be).
Pat the mackerel dry and place it on a chopping board, skin side up. Carefully peel off the film-like layer of skin, keeping the thicker skin intact.
Line a sushi mat with a sheet of plastic wrap and lay one of the fillets, flesh side up, in the middle of the mat. Lay half the shiso leaves on top, if using, then shape half the rice into a log to sit along the inside of the fillet. Lift the mat to roll the sushi and shape firmly so that the rice and fish stick together. Unroll the mat and repeat with the second fish fillet.
Cut each mackerel fillet into eight to ten equal pieces and serve with soy sauce.
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