4poundsplumswashed, cut in half and pitted (or amount needed to equal 8 cups of food-processed chopped plums)
3/4cupchopped onionabout 1 medium
2clovesgarlicminced, about 2 teaspoons
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. Turn off heat, remove lid and let jars sit in canner to cool for 5 minutes. Remove to a cloth to cool completely.
Test lids, label jars, store and use within a year to 18 months.
This recipe was adapted from one in the Ball Blue Book of Preserving, I decreased the sugar a bit and kept the safe canning ratio by omitting two fresh ingredients (fresh chili peppers for dry flakes and fresh ginger for dry) so I could add another teaspoon of fresh garlic. I also changed/added other dry ingredients which doesn't affect acidity to create a product close to the Asian plum sauces I like.