wasabi
A sharp green paste made from the rhizome of the wasabi plant, served in Korea mainly with raw fish.
Wasabi paste in tubes or as a powder is sold in Asian shops and often in supermarkets alongside sushi ingredients, though it is usually mostly horseradish and mustard coloured green. Fresh wasabi rhizome is rare and expensive and mostly found through mail order or well-stocked Japanese shops.
Substitute:
Freshly grated horseradish, with a little hot mustard if you like, comes very close to the usual tube paste, but it is harsher and coarser than real wasabi and lacks its delicate, slightly sweet herbal note.