wasabi
A pungent green condiment made from the rhizome of the Japanese wasabi plant, usually served as a paste with sushi and sashimi.
Wasabi paste in tubes or as powder is sold in many supermarkets and every Asian grocery, but it is usually made mostly of horseradish, mustard and colouring. The real rhizome is only occasionally found fresh in well-stocked Asian shops or by mail order; freshly grated it is milder, sweeter and greener in aroma, and its heat fades within minutes.
Substitute:
Freshly grated horseradish with a little hot mustard matches the nose-clearing heat, but it is harsher and lacks the sweet, herbal note of real wasabi.