Every Seattle Japanese restaurant will provide wasabi with each order of sushi. The fiery green condiment is made from its namesake plant, which is distinct from, but related to, horseradish. This plant–grown almost exclusively in Japan–is extremely challenging to cultivate, and may be prohibitively costly to serve in other countries.
Consequently, nearly all Japanese restaurants in Seattle and the rest of the Western world offer a horseradish paste that has been colored to approximate wasabi in appearance, and is known in Japan as Western wasabi. The pairing of wasabi with sushi dates back quite some time, and while the initial inspiration is uncertain, some suggest that it may have helped keep sushi safe to eat (due to anti-microbial properties) before today’s refrigeration and safe handling practices existed.