Korean hot mustard paste
A sharp paste of ground mustard seed, usually sold in a tube, whose heat rises into the nose rather than burning on the tongue.
Korean mustard paste (yeon-gyeoja) comes in tubes, or as a powder (gyeoja-garu) to mix yourself, from Asian grocers or by mail order. Unlike everyday table mustard it is barely sour and not sweet, but piercingly hot in the way wasabi is.
Substitute:
English mustard powder, mixed with a little lukewarm water and left to stand for a few minutes, comes closest in heat. Dijon or German mustard is too mild and brings vinegar and sweetness that change the dish.