Put the mustard seeds in a sealable container and fill with filtered water 2 inches above the mustard seeds. Let the seeds soak and plump in the refrigerator for at least 3 days.
Strain the liquid off and rinse under cold water in a mesh strainer. The seeds will have a fish egg texture; this is exactly what you want.
Mix the mustard seeds, vinegar, wine, 2 1/2 tablespoons salt, the garlic powder and sugar together in a large bowl. Use a blender to blend the mustard, but only about a cup at a time for about 20 to 30 seconds each time (in order to get a good puree as well as save your blender motor). This first puree will not get the seeds broken down much, and the result will be a coarse-ground mustard that will need to sit another day to soak. Blend the mustard again the next day, and let it sit again for another day. On the third blend, you should find it is ready to use, although you can blend as many times as you want to get the texture you desire. For the adventurous, add the xanthan powder if desired to increase the textural balance. Taste and adjust the salt as needed to balance the vinegar.
Use on your favorite sausage, or to make a wonderful vinaigrette.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.