Although most of us think of mustard as a prepared paste that comes out of a jar, it starts life as something much more basic. The small seeds that go into the many types of mustard products come from a plant that is also grown for its green leaves. These greens are popular in Chinese cooking and their flavour bears a distinct but much more subtle resemblance to the more powerful seeds.
Mustard seeds are used whole (when they are usually lightly toasted or fried to bring out the flavour and fragrance), made into a dry powder, pressed for oil or pounded into a paste or spice blend. The seeds come in white, brown and black varieties. The black seeds are popular in Indian cookery while the white and brown seeds are the type most commonly made into prepared mustard pastes. All varieties contain small amounts of a powerful compound called isothiocyanate, whose sinus-clearing properties are also found in other flavourings and plants such as wasabi and horseradish. More insidiously, isothiocyanate has also been used as a chemical weapon in the form of mustard gas. In addition to its culinary uses, mustard is believed to have medicinal properties, including relieving arthritis, toothaches and digestive problems.
There is a huge variety of prepared mustards with flavours ranging from mild to throat-searing, and whose subtleties (or lack thereof) depend on the amount and type of other ingredients they are blended with. The prepared mustards are made by grinding the seeds (although they are sometimes left whole) then mixing them with other ingredients such as vinegar or wine, honey or other sweeteners, and spices and herbs such as tarragon, turmeric or chillies. The usual uses for these mustards are as a sandwich spread or condiment for charcuterie and fresh meats.
For powdered mustards, the seeds are ground then mixed with an extender such as flour. The powder is reconstituted with water before using.
Except for the greens, mustard isn't popular in traditional Chinese cuisine, although the dry mustard powder (almost always Coleman's brand) has a few uses. In old-fashioned Chinese tea houses the reconstituted mustard powder is sometimes served with soy sauce as a dip for dim sum. It's good in the cold Shanghainese dish of gai see fun pei. Blanch mung bean sheets in hot water until tender then slice in ribbons about 5mm wide. Slice cucumber into julienne strips. Mix hand-shredded poached chicken with a mixture of soy sauce, Shanghainese rice vinegar, sesame paste, sesame oil and a little reconstituted mustard powder to taste then toss everything together. For a delicious salad, mix shreds of poached chicken with soy sauce, oyster sauce, sesame oil and reconstituted mustard powder. Toss with finely sliced iceberg lettuce, spring onions and toasted sesame seeds.