Blend all of the ingredients in a food processor or blender.

Spoon mixture into the top of a double boiler. Cook over simmering water until thickened, about 10 minutes, whisking often to prevent curdling.

Let the mustard cool to room temperature. Store in the refrigerator, in a tightly sealed container.

Serve as a condiment for meats and deli sandwiches. This mustard keeps for up to 2 weeks.