Place all the ingredients in the container of a blender, blend for 1 minute. Scrape down the sides of the blender with a rubber spatula and process for 30 seconds longer. The mustard should be a little thicker than heavy cream. Store in glass jar, well-sealed, in the refrigerator and let the flavors marry for at least a day before using. The longer it sits, the better it tastes; it also will get thicker and more yellow.