Umeboshi Onigiri is a traditional Japanese rice ball that is filled with pickled plum, known as umeboshi. To make Umeboshi Onigiri, start by cooking the sushi rice and letting it cool slightly. Take a small handful of rice and shape it into a triangle or oval shape. Make an indentation in the center and place a piece of umeboshi inside. Close the rice ball and wrap it with a strip of nori seaweed. Umeboshi Onigiri has a unique and tangy flavor due to the pickled plum filling. It is a popular choice for Onigiri lovers who enjoy a slightly sour and salty taste. Umeboshi Onigiri can be enjoyed as a snack or as part of a traditional Japanese meal.