Step 1

Chop plums in a food processor and transfer to a large heavy-bottomed pot (it should equal 8 cups chopped plums). Chop onions in processor and add them to the plums.

Step 2

Combine the remaining ingredients with plums and onions, bring to a boil and then reduce heat. Cook until thickened, about an hour.

Step 3

For a smooth sauce, puree with an immersion blender right in the pot (optional if you'd like a chunkier sauce, though it helps speed the cooking time) and continue cooking for another hour, or until the sauce reaches the desired thickness.

Step 4

Ladle sauce into hot jars, leaving 1/4-inch headspace. Attach lids and rings. Process 20 minutes in a boiling-water canner. Turn off heat, remove lid and let jars sit in canner to cool for 5 minutes. Remove to a cloth to cool completely.

Step 5

Test lids, label jars, store and use within a year to 18 months.