Mackerel spoils fast. That is why most of Japan did not eat it raw. Hakata did, and has for a long time, because what comes out of the Genkai Sea reaches the market while it is still new.

There is a second reason, and it is biological. The parasite carried by mackerel on the Sea of Japan side is much less inclined to migrate from the gut into the flesh than the one on the Pacific side. Eating mackerel raw in Kyushu is therefore a good deal safer than the same act would be elsewhere, and the custom took root here for reasons nobody at the time could have explained.

The dish itself is simple. Mackerel sliced a little thinner than for ordinary sashimi, dressed with soy sauce, mirin, and ground sesame.

Wasabi, shredded nori, fine ginger alongside. It goes onto hot rice, or into ochazuke with tea poured over it. The name is younger than the practice: gomasaba first appears in print in 1984, in a book on Fukuoka cooking, under the heading mackerel in sesame soy sauce. Mackerel is landed all year, though the fat comes up from autumn into early spring. What the rest of the country treats as a risk is here just lunch, eaten without any particular ceremony.