When a Japanese recipe calls for mustard, regular prepared mustard shouldn’t be used. Unlike American mustard, which is a combination of mustard seeds, vinegar, and other spices, Japanese mustard is only made of powdered mustard and water.
You can either buy powder type or ready to use tube type. When you prepare mustard on your own from powder, mix a small amount of water with mustard to form paste in a shot glass. Cover it for a while to develop potency. Once it’s made, it lasts only for a day or so. It won’t get bad, but it gets dry and lose potency. So that prepare a small amount of mustard every time you need.
Japanese mustard is served on the side, to accompany natto (fermented soy beans), oden, tonkatsu (breaded and fried pork), etc. Chinese and other asian cuisine use the same powdered mustard.
Japanese grocery stores carry both powder and tube type of mustard. Korean and Chinese grocery stores may carry tubes. For those who wish to order online, here are the links;