Pour both the seeds and liquid into the container of a blender or food processor fitted with a steel blade. Process with several on-off motions until the seeds are bruised and broken.
Add the dry mustard, salt, allspice and water, and process for 30 seconds.
Scrape down the sides of the container with rubber spatula and process for 30 seconds longer.
Keep in a well-sealed container and allow to stand overnight before using.