4poundsplumswashed, cut in half and pitted (or amount needed to equal 8 cups of food-processed chopped plums)
3/4cupchopped onionabout 1 medium
2teaspoonsred pepper flakesor to taste
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.
Combine the remaining ingredients with plums and onions, bring to a boil and then reduce heat. Cook until thickened, about an hour.
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.
Ladle sauce into hot jars, leaving 1/4-inch headspace. Attach lids and rings. Process 20 minutes in a boiling-water canner. Remove to a cloth to cool completely.
Test lids, label jars, and use within a year to 18 months.
This recipe was adapted from one in the Ball Blue Book of Preserving, keeping the safe canning ratio by decreasing the sugar and omitting two fresh ingredients so I could add another teaspoon of fresh garlic. I also changed some of the dry ingredients which doesn't affect acidity.