Cook. Turn on you kitchen vent fan or better still, make this outdoors on your grill. Remember, mustard gas is right up there with pepper spray. Whisk everything except the vinegar together in a non-reactive pot and simmer over medium to low until it thickens, about 30 minutes. Stir often so it doesn't burn.
Stir in the vinegar and simmer some more until it is the thickness you like. Then pour it into a clean bottle and refrigerate. It will be pretty potent for a few days, and then it will mellow and keep for months.
Serve. Use as you would your usual store-bought mustard.
