In Japan, there is a variety of plum called ume. It actually resembles an apricot more than a plum, but it's called a plum. It's very astringent and when eaten raw can give a person a stomach ache, so the Japanese infuse them in alcohol and pickle them. In pickled form, these are called umeboshi.Umeboshi are often combined with shiso leaves, so it's common to find dark purple shiso leaves with them, as shown in the photo above. These plums are very tart and salty, and if it is your first time trying them, do not pop the whole thing in your mouth! You will end up with a pucker face and your friends will point at you and laugh.
Umeboshi are often served on top of a bowl of rice, and this is a wonderful combination. The red circular umeboshi in the white rice is meant to symbolize the flag of Japan. I eat this simple dish at least once a week, taking little bites of umeboshi and scooping in mouthfuls of rice. I also enjoy umeboshi combined with shiso in maki sushi rolls.
These are easy to find in large chain grocery stores, Asian groceries, and online. Try some!