Warning: Mixing wasabi and karashi paste may cause an explosion. Only a joke of course, but I’ve gotten a few laughs out of that.
Karashi is a mixture of ground mustard seeds and horseradish – like wasabi- a little goes a long way. So unlike European mustard, it’s not an emulsion based with vinegar. It’s normally sold in a dried powder form (just add water), or as a paste in a tube.
Karashi paste is served as a condiment with dishes such as tonkatsu, steak and oden. It is also used in sauces based with miso, mayonaise and sometimes in sunomono (things with vinegar) dressing.