Soak the mustard seeds in the beer overnight. About 20 minutes before you are ready to make the mustard, stir the mustard powder, minced onion and thyme into the soaked seed mixture and allow to sit.
Place the mustard mixture to a blender along with the vinegar and salt. Grind until it is the consistency of a paste, with some seeds remaining visible. This may take a minute or two. Ladle into quarter pint jar and process in water bath for 15 minutes.