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Karashi (辛子, からし) is a Japanese hot mustard made of a mixture of crushed mustard seeds of Brassica juncea and horseradish. It’s used as a condiment or as a seasoning in Japanese cuisine, such as oden, gyoza, and tonkatsu. Karashi is usually sold in powder form (see the photo above) or paste form in tubes (see the photo below).
Karashi is a spicier and more potent condiment than mild yellow mustard.
To use karashi powder: Mix 1 teaspoon of karashi powder with 1/2 tablespoon of hot water in a small bowl. Cover the bowl with plastic wrap, and flip upside down to let it steam under the bowl for 5 minutes to get good taste and fragrance. Unwrap the bowl and ready to use.
Substitute: The closest substitute would be English mustard and Chinese hot mustard powder.