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.
Notes
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 Asian plum sauce.Altitude Adjustment: Higher altitudes will have to adjust and add processing time according to this chart.If you have any jars that don't seal (which is rare!), store them in the fridge and use first.