Wagarashi is a very sharp tasting Japanese mustard made from brown mustard seed (Brassica juncea.)
English mustard uses brown mustard seed and white, so it approaches Wagarashi in heat; the mild American-style yellow mustard uses just white mustard seed.
It is used as a seasoning, often served on the side in small quantities for you to mix into a dish such as noodles.
It is sold both powdered in a box and prepared in tube. The tube more popular.
Wagarashi loses flavour quickly when exposed to air.
Cooking Tips
To mix the powder, mix it in a small cup (about the size of a sake cup) with a few drops of hot water until you get a thick paste that sticks to the bottom, then invert the cup for about 1/2 an hour.
This allows the taste to develop, while protecting it from air.
Substitutes
A hot mustard, such as English mustard.