Karashi is certainly not the most important of Japanese pantry items, but it is fairly common and easily stored. As such, it tends to be available at any Japanese grocery store and many Asian grocery stores.

Because true karashi is simply powdered brown/Indian mustard, it can also be made at home by pulverizing brown mustard seeds in a spice grinder or with a mortar and pestle. If you opt to go this way, make sure to look sift through the seeds to look for broken pieces and small rocks that may end up mixed in. Brown mustard seeds are often available at Indian grocery stores, and some conventional grocery stores.

Look for karashi powder or paste that lists mustard as an ingredient (or mustard and water in the case of the paste). Dried mustard, sold in small tins, will generally be without additives or additional ingredients. Mustard paste, sold in small tubes, may contain additives/preservatives other than mustard and water.

The pastes are convenient, but karashi mustard is extremely easy to prepare from powder and can be stored for a long period in this state. For these reasons, I personally recommend using the powdered form.

Prepared karashi (karashi powder mixed with water) is popularly used as a condiment alongside dishes like tonkatsu (breaded fried pork cutlets), dumplings, oden (simmered fish cakes), and natto (sticky fermented soy beans). It can also be added to dressing, sauces, and marinades, or used to make pickles (e.g. nasu no karashi zuke, aka mustard pickled eggplant).

When added to sauces or marinades, karashi can be used in its prepared or powdered form depending on the desired heat and thickness of the final dish. That being said, most Japanese preparations call for it to be mixed with water and made into a paste before combining with other ingredients.

Prepared karashi can be mixed with other ingredients to make other condiments as well.  Karashi mayonnaise (that’s the Japanese name and the description) partners well with meats (e.g. wafu hambagu) and vegetables. Karashi, vinegar, and miso combine to make karashi sumiso, a dressing/sauce that tastes great alongside vegetables, seafood, and rich meats like duck.

Preparation

Powdered karashi is very simple to work with. Simply spoon out a portion of mustard and add an equal amount of lukewarm water, then mix with a chopstick or other wooden utensil to form a coarse paste (some sources claim that using metal utensils will dull the flavour). At this point, the mustard is ready to use. Thin mustard can be made by using a 1:1.5 ratio of mustard and water. One teaspoon of karashi powder will yield approximately one teaspoon of finished paste.

Karashi powder can also be added to sauces and marinades, or made into a paste and then added, depending on the desired thickness of the end product (adding powder alone will yield a thicker product).

Karashi sold as a paste is ready to be used as-is.

Storage

Powdered karashi mustard should be stored in an airtight container (ideally the tin it comes in) and away from heat or moisture. It has a very long shelf life, though it may loose some of its potency over time.

Karashi paste should be refrigerated to preserve its freshness. Once opened and exposed to air, it will only last for about a month before losing much of its flavour.

There is not a great deal of variation within karashi mustard, though it is worth differentiating it from other mustard varieties.  For more on this, see the ‘Substitutions’ section below.

Karashi is made with brown mustard seed (Brassica juncea) which has a good deal more heat than its relative white mustard, (Sinapis alba).  Additionally, the use of water alone during preparation creates a much hotter end product, as vinegar (commonly used in European mustard preparations) neutralizes the alkaline ingredients that give mustard its spicy heat.  Because of these two factors, substitutions should be chosen carefully.

Prepared yellow mustard (and offshoots like Dijon mustard) is made with a milder seed, vinegar, and in some cases other ingredients or spices, making it a poor choice when looking for a karashi substitute.  Prepared brown mustards are made from a hotter mustard seeds, but are still generally made with vinegar and some spices.  As a general rule, prepared mustards (i.e. those sold as pastes) are not good replacements for karashi.  The one exception would be prepared hot English mustard, though even this has been salted and sweetened slightly.

Hot English mustard powder is similar to karashi in both flavour and preparation, though it is made with a combination of yellow and brown mustard seeds so it doesn’t pack quite as much punch.  English mustard is easily found in metal tins (powdered) or small glass jars (prepared) at most grocery stores – Colman’s is the most famous brand.

Chinese mustard powder is the closest relative to karashi (both in terms of flavour and biology) and the best substitute.  If you are unable to find karashi but you do have access to Chinese powdered mustard (easily found at Chinese grocery stores), you can use it interchangeably.

Finally, it’s worth noting that if you are unable to find karashi or Chinese mustard powder, you can always grind your own brown mustard seeds in a spice grinder. The resulting powder will be virtually identical to store-bought karashi, and certainly fresher.