COMBINE beer and brown mustard seeds in a medium saucepan. Bring to a boil. Remove from heat, cover and let stand at room temperature until seeds have absorbed most of the moisture, about 2 hours.
PREPARE boiling water canner. Heat jars and lids in simmering water until ready for use. Do not boil. Set bands aside.
PLACE mustard seeds and remaining liquid in a food processor or blender. Process until chopped and slightly grainy.
TRANSFER mixture to a large saucepan. Whisk in water, vinegar, brown sugar, dry mustard and onion powder. Bring to a boil. Reduce heat and simmer, stirring frequently, until volume is reduced by a third, about 15 minutes.
LADLE hot mustard into hot jars leaving 1/4 inch headspace. Remove air bubbles. Wipe rim. Center hot lid on jar. Apply band and adjust until fit is fingertip tight.
PROCESS in a boiling water canner for 10 minutes, adjusting for altitude. Remove jars and cool. Check lids for seal after 24 hours. Lid should not flex up and down when center is pressed.