I have just returned from a dinner in a Japanese restaurant where I tasted Wasabi.
I would like to know which are the ingredients of this sauce, which is the recipe and if it easy to make.
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This is remarkably simple.
Simply take some wasabi powder and mix with equal parts water adjusting for desired consistency.
Update - Regarding fresh wasabi. As indicated in my comment below fresh wasabi is very rare and not often seen outside of Japan. It is very expensive and is considered a delicacy. Nearly any wasabi you'll find in North America, and likely Europe too, is going to be made from a wasabi powder. The powder is typically a combination of low quality wasabi (if any), regular horseradish, mustard, cornstarch and food coloring.
If you do get your hands on fresh wasabi then you would grate it with a very fine grater. Typically sharkskin or ceramic. The fresh root would provide some natural moisture so you may not need to add much water to adjust consistency, if any.
I think genuine wasabi root is gaining more of a foothold in the states. I found packaged grated wasabi root at Whole Foods market last time I was in the sushi section. Fresh wasabi has a wholly different flavor profile IMHO, much sweeter and without that nasal-clearing quality of horseradish.