When a Japanese recipe calls for mustard, regular prepared mustard shouldn’t be used. Unlike prepared mustard, which is a combination of mustard seeds, vinegar, and other spices, Japanese mustard is powdered mustard seeds dissolved with a small amount of water to form paste, each time before using. The prepared mustard paste is kept covered so as not to lose potency and moisture. It’s pretty spicy compared to regular prepared mustard, such as Gray Poupon.
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 sell prepared Japanese mustard in a tube.
Japanese grocery stores carry both powder and tube form of mustard. Korean and Chinese grocery stores may carry tubes. For those who wish to order online, here are the links;