Even in a country renowned for its bizarre dishes, frozen sashimi – yes, eating frozen raw fish – manages to be one of Japan’s most surprising culinary specialities. It is a legacy of the Ainu, the indigenous people often ignored by the rest of Japan, and originates in Hokkaido, the country’s northernmost and mountainous island.

Ruibe is half-frozen sashimi, usually wild salmon, hauled from Hokkaido’s cold waters. The fish was traditionally frozen in the open air then overwintered under the snow, before being dug up to eat in spring …and the Ainu didn’t wait for it to thaw. It’s literally a melt-in-the-mouth dish.

Nowadays, chefs and home cooks choose salmon “as fat, handsome and fresh as possible”, then simply clean, fillet and freeze it in the freezer, explains chef Shinichi Maeda, who runs An Dining at Ki Niseko in Abuta-Gun, Hokkaido. The ruibe is then cut and eaten while still frozen, and served with soy sauce and fresh wasabi.

“Ruibe’s unique point is its half-frozen texture,” says Yuichi Kamimura, another Hokkaido-born chef and owner of Kamimura restaurant, also in Abuta-Gun. “It is somewhere between cubed ice and the silkiness of sorbet.”

Not surprisingly, ruibe translates as “melted food”. Not generally expensive, ruibe can be found on the menus of some of Hokkaido’s izakaya – the casual, local restaurants ubiquitous throughout Japan. But rather like the Ainu and their culture, one has to look hard to find it. A few restaurants in Sapporo, Hokkaido’s largest city, list the local speciality on their menus: try Suginome (suginome.jp), which has various branches, or Hyousetsu-no-mon (hyousetsu.co.jp).