Aizu has a vessel made specifically for this. A nishin-bachi, a rectangular bowl sized to the length of a dried herring fillet, produced in Aizu Hongo ware.

The method: line the bowl with sansho leaves. Scrub the herring hard with a brush, take the heads off, and pack them in tightly. More sansho leaves, more herring, and so on in layers. Cover the top with leaves, pour in vinegar, sake, and soy sauce, put the lid on, and weight it down. After two or three days it can be eaten, by which time the sansho has gone all the way into the flesh.

The reason any of this exists is that in the Edo period almost no fresh fish reached Fukushima at all. What did reach it was migaki-nishin, herring caught off Hokkaido and dried hard for transport. It kept more or less indefinitely and it delivered protein, and in a landlocked province those two properties were worth a great deal more than freshness.

The detail that tells you how seriously this was taken is the bowl. A region does not commission a specific ceramic form, from its own kiln, in a specific rectangle, sized to one ingredient, for something it makes once in a while. In some households it was part of what a bride brought with her when she married in.