Originally from Dijon, France, Dijon mustard is a condiment made from mustard seeds, white wine, grape must, and seasonings. Besides its obvious use as a sandwich condiment, Dijon is the secret ingredient in countless sauces. It's especially crucial in vinaigrettes as an emulsifier, meaning it helps keep the oil and vinegar from separating.
When the mustard jar is just about empty, don't throw it out. Instead, add some oil and vinegar to the jar and shake into a mustardy vinaigrette.
Substitute another prepared mustard, or 1 tsp. dry mustard per 1 Tbs. Dijon mustard called for. You may also want to add a dash of white wine.
Most supermarkets offer a bewildering array of Dijon. In France, there are federal standards for what constitutes Dijon mustard, but in the U.S. there are none. Both Maille, from France, and the popular Grey Poupon, are excellent choices.
Dijon will keep indefinitely in the refrigerator.