Called gari in a sushi restaurant, or beni shouga outside, I made my own sushi ginger over the weekend. Steeping ginger in the expressed juice of umeboshi salted plums is the traditional method, although basically nobody does this these days commercially – the poor ginger is simply steeped in a store-bought chemical mix and then served as-is. To make gari in the traditional way, first off you need the ume-vinegar or liquor, a by-product of making your own umeboshi plums, and some young stem ginger.
Then it is simply a matter of slicing the ginger thin, drying it partly then curing it in the ume-vinegar. The resulting product is sushi ginger. A true gourmand will know it for its palate-cleansing properties, where one should eat a small quantity between different types of sushi to ensure you enjoy the taste of each. Shredded fine, it is also a mainstay of many Japanese dishes like home-style yakisoba, okonomiyaki and various Kansai treats such as takoyaki.