Kari Leaves: Olive-green kari leaves (also called curry leaves), a distant cousin of the citrus family, have a delicate aroma and flavor and are available in the produce section of Indian grocery stores. They last up to 3 weeks in the refrigerator or in the freezer for up to a month. Do not use the dried (and highly insipid) version of these leaves. If unavailable, omit from recipe.
Mustard Seed: The black variety is slightly stronger than the more commonly available yellow kind, the source of ground mustard used in American kitchens. South Indian cooks pop them in hot oil, like popcorn, to extract an unusually sweet and nutty flavor that is crucial to this region’s foods.
Tamarind: Highly acidic, tart and complex-tasting tamarind fruit is used extensively in Southern Indian cooking. The pulp can be extracted and stored in paste form as tamarind concentrate. It is widely available in Indian grocery stores and other ethnic supermarkets. It will keep in a covered container in the refrigerator for up to 1 year. Lime juice is an acceptable substitute.
Urad Dal: These split black lentils have an off-white interior. This legume is crucial to the cuisine of southern India, and is the base of many batters for steamed cakes, dumplings and crepes. They are also used as a spice to flavor oils; when roasted, they are blended with other "traditional" spices to provide an essential nutty flavor. Look for them in Indian groceries and in natural-foods stores. They will keep in an airtight container in a cool, dry place for up to 1 year.