Combine all the ingredients, except the horseradish, in a stainless-steel bowl. Cover, and let stand for 2-3 days
Put the ingredients in a blender and whirl until as smooth as possible.
Add a couple tablespoons of water if the mustard until it reaches a desired consistency.
Add the horseradish, and pulse a few times. Taste and adjust the flavors as the mustard ages
The mustard will keep for up to 6 months refrigerated, although it’s best if used within one month