After black pepper, mustard is the most-consumed spice in the United States. Outside of condiments and pickling, however, it’s sparsely used.
The seeds have featured heavily in Indian cuisine for millennia, and that's likely how they migrated to Japan, piggybacking on the spread of Buddhism from the Asian mainland. Whether karashi (mustard) was brought as a culinary ingredient or curative is unclear — what’s clear is that Japan loves it and loves it hot. Without additional acid or heating, as in other preparations, karashi’s innate piquancy is untempered.
Mustard can be found in Japan-based cuisines dabbed aside chūka (Chinese) and yoshoku (traditional Japanese) dishes like shūmai dumplings or breaded tonkatsu (pork cutlet) or in classic Japanese arrangements like nattō (fermented soybeans) and oden (winter stew of various ingredients). The Kumamoto specialty — karashi renkon — combines mustard with miso stuffed into lotus root hollows then fried.