Wasabi is a a rhizome similar in flavor and heat to horseradish that is used dried and ground or grated as a condiment with sushi, sashimi as well as other Japanese noodle dishes.
Wasabi is quite strong when it is first grated but the heat diminishes as the wasabi is exposed to air.
Dried, powdered wasabi (real wasabi not horseradish powder and dry mustard) is also available but just like its fresh counterpart it is hard to find and quite expensive.
Wasabi powder is combined with water to form a thick paste. Wasabi is typically mixed with soy sauce when being used with sushi or sashimi. (Nutritional information is for raw, wasabi root).
Most all wasabi served in the U.S. is not wasabi at all but a powder made from dried horseradish, food coloring and dry mustard. This product is easy to find in most well-stocked grocery stores and of course Asian markets.
Japanese restaurants found that fresh wasabi was not preferred by Americans so the horseradish mixture has become the standard. In the traditional fashion fresh wasabi is grated using a small shark-skin grater.
Per 1 tablespoon fresh wasabi, use:
1 tablespoon wasabi paste OR> 1 tablespoon wasabi powder or hot mustard powder* (such as Coleman's) mixed with water or use equal parts of well-drained prepared horseradish. OR> 1 tablespoon wasabi powder mixed with 2-3 teaspoons of water, allow to sit for 1-15 minutes before using.
*Warning. Beware of Chinese Mustard Powder, it is exceedingly hot.