Umeboshi are salt-pickled ume, the apricot-like fruit of Prunus mume, cured until they collapse into soft, intensely sour-and-salty wrinkled spheres. A cornerstone of the Japanese larder, they are eaten in tiny amounts — one tucked into a bowl of rice, folded into an onigiri, or sliced as a relish — for a flavour so puckeringly acidic and saline that a single fruit can season a whole meal.