Japanese ume — a sour apricot, not a plum — salted and sun-dried, often with red shiso for colour. Fiercely sour and salty: the centre of an onigiri, the red dot on a lunchbox of rice.
Japanese grocers. Wakayama (Kishu) ume are the most prized. The brine is sold as ume vinegar (umezu).
Worth knowing
Stone one and mash the flesh with a little mirin and soy for a dressing for cucumber, chicken or cold noodles. It's salt as well as acid; season after.