Purée plums and syrup in a food processor or blender.

Pour purée into a saucepan and mix with remaining ingredients. Let simmer for about 30 minutes, until thickened.

Allow to cool, then keep refrigerated in a sealed container.

In large heavy saucepan, bring plums, onions, water, ginger and garlic to boil over medium heat. Cover and lower heat to simmer, stirring occasionally, until plums and onions are very tender, about 30 minutes.

Place a large sieve over a large bowl, and pour mixture into sieve. Using a potato masher, or the bottom of a cup, press the mixture through until you have extracted as much of the liquid as you can.

Pour liquid in bowl into a clean pot and add remaining ingredients. Taste, and adjust any spices as necessary. Bring to boil, stirring occasionally, then reduce heat and simmer until mixture has thickened, about 45 minutes.

Allow to cool, then keep refrigerated in a sealed container. Or can fill hot sauce into canning jars, seal, and process in a boiling water bath for 30 minutes.