Most condiments, including ketchup and relish, are relatively simple things, but mustard is much more complex. The familiar bright-yellow variety served on hot dogs barely resembles more upscale versions, such as French-made, brand-name Moutarde de Meaux. Ballpark mustard has an unsubtle tartness and straightforward taste, while the French mustard is relatively mild and has a rich, complex flavor. You can use substitutes in a recipe calling for Moutarde de Meaux, but plain yellow mustard isn't one of them.
Mustard-based condiments are as varied as people. They're made from the seeds of various plants in the brassica family, cousins to cabbages and many Asian greens. The seeds come in white, yellow, brown and black versions, with the light-colored seeds yielding milder condiments while the darker ones pack a more assertive punch. Freshly ground mustard seeds mixed with water make a hot and pungent paste, so commercially prepared mustards are usually mellowed with spices, sugars, wine, vinegar, fruit juice, honey or other ingredients.
Moutarde de Meaux
Moutarde de Meaux is an especially complex example of the mustard-maker's art. It's a style of mustard known in France as "a l'ancienne," or old-fashioned. Meaux mustard has been produced since the mid-1600s, so the term is well justified. It's made with a blend of different mustard seeds soaked in wine and vinegar or verjus, the tart juice of unripe grapes. The softened seeds are coarsely ground and many are left intact in the finished condiment. Its flavor resembles a relatively mild Dijon mustard, with a distinctively chewy and grainy texture from the whole mustard seeds.
Genuine Moutarde de Meaux can be difficult to find if you live outside a major city, though you can order it from many online stores. If you need something to use immediately in a recipe, you can achieve a similar flavor with any good-quality Dijon mustard. It will be less complex than a true Moutarde de Meaux, and lack its texture, but the flavor will be appropriate. A grainy Dijon mustard is even better, because it's closer in texture to the authentic product. Avoid German-style grainy mustards, which are invariably either too hot or too sweet to serve as a good substitution.
Making Your Own
If you have time on your hands, you can conjure up a usable simulation of Moutarde de Meaux in your kitchen. You probably won't be able to find verjus unless you live in a wine-producing region, but a good white wine vinegar is an appropriate alternative. Soak equal parts of black and brown or yellow and brown mustard seeds in twice their volume of wine vinegar and dry white wine. After two days of soaking, take out one-fourth of the seeds and process the rest of the mixture to a coarse paste in your blender. Stir in a pinch of salt and the whole seeds, then transfer your mustard to a sealed jar and refrigerate it for a few more days to let the flavors mellow.