In a blender or food processor, combine all ingredients; cover and process for 1 minute. Scrape down the sides of the container and process for 30 seconds.
Transfer to a small saucepan and let stand for 10 minutes. Cook over low heat until bubbly, stirring constantly. Cool completely. If a thinner mustard is desired, stir in an additional 1-2 tablespoons water. Pour into small airtight containers. Store in the refrigerator.