In a blender or food processor, process the plums, onion and garlic in batches until smooth. Transfer to a Dutch oven. Stir in the remaining ingredients. Bring to a boil. Reduce heat; simmer until reduced by a third, 60-90 minutes.
Carefully ladle hot mixture into hot sterilized half-pint jars, leaving 1/4-in. headspace. Remove air bubbles and adjust headspace, if necessary, by adding hot mixture. Wipe rims. Center lids on jars; screw on bands until fingertip tight. Place jars into canner with simmering water, ensuring that they are completely covered with water. Bring to a boil; process for 15 minutes. Remove jars and cool.
Canning Altitude Jam-Jelly
The processing time listed is for altitudes of 1,000 feet or less. Add 1 minute to the processing time for each 1,000 feet of additional altitude.