Except that it wasn't - not real wasabi, anyway.
is a plant that belongs in the cabbage family, which includes horseradish and mustard. Native to Japan (although it's now cultivated in other countries), wasabi is expensive, primarily because it is difficult to grow - it needs cool, clean, mountain waters. When the flesh of the stem is pierced, usually by grating, it releases volatile compounds, the most obvious (and notorious) of which has a sinus-clearing pungency. But the effect is much more subtle and complex than the one that comes from grated horseradish, which is far cheaper. Most of the wasabi you get in inexpensive to mid-range Japanese restaurants is, in fact, horseradish (sometimes combined with mustard) that's been tinted green, as is the wasabi paste that comes in tubes, and the powder that's reconstituted with water.
Real wasabi is best enjoyed fresh - and not just the root; the paste should be prepared just before it's consumed. It takes just a few minutes before the volatile compounds in wasabi reach their peak and start to subside, although their release can be slowed if the grated paste is covered, to reduce exposure to air.
Traditional wasabi graters are made of shark skin - a roughly-textured, nubbly surface that doesn't cut the stem, as a metal grater with sharp "teeth" would; instead, the shark skin reduces the wasabi to a fine, moist paste. The first indication that you're being served real wasabi is fine shreds in the paste; the second is the flavour. Yes, you still taste the sharpness with faux wasabi, but it fades quickly on the palate, giving way to much more complex and varied flavours.
