Sashimi of horse mackerel or yellowtail, steeped in a sauce of soy, sake, mirin, sesame, and ginger. It sits in that for a while and comes out darker, firmer, and considerably more savory than a piece of raw fish has any particular right to be.

The name is unresolved. One account says fishermen from Oita learned the method from fishermen in Okinawa, the old kingdom of Ryukyu, and brought it home under that name. Another says it comes from rikyu-ae, the term for a sesame dressing, worn down over time. Both explanations circulate freely in Oita and neither one of them has ever won the argument.

What it was for is clearer. This used the offcuts, the trimmings left after a fish had been broken down for sashimi, and whatever raw fish had not sold. It was crew food, made on the boats themselves, and it was also a practical way of keeping fish edible for another day at sea. From the southern coast of the prefecture it spread outward until the whole of Oita was making it.

Now it goes onto hot rice, or into a bowl with tea poured over the top of it, and restaurants list ryukyu-don near the top of the menu. A dressing invented to deal with scraps has ended up being the thing that people order first.