Just like how real Parmigiano-Reggiano comes from the towns of Parma and Reggio in Italy and real Champagne hails from Champagne, France, the real stuff called Dijon is made right near the Burgundy town of Dijon. In fact, concern for protecting the name and quality of its mustard caused the city of Dijon to impose rules of production for its mustard in 1634.
The firm of Edmond Fallot has been making mustard in Beaune, south of Dijon, since 1840 and fully subscribes to the rules that make this mustard magnificent: they choose very high-quality mustard seeds and mill them with traditional grindstones that don't create a lot of heat (which drives away flavor). You can taste the results in mustards that have a fair bit of nose clearing heat married to a complex, earthy flavor. Use them on sandwiches, in vinaigrettes or mixed into marinades.
A spoonful of mustard can add an unusual depth of flavor to sauces, soups and stews. To preserve the mustard's pungency and power, add it at the end of your cooking.