Combine the first 4 ingredients in a small saucepan. Bring to a boil; cover, reduce heat, and simmer for 10 minutes. Uncover and cook for 3 minutes or until liquid almost evaporates, stirring frequently.
Place the onion mixture in a bowl, and cool to room temperature. Stir in the plum and remaining ingredients. Cover and chill.
I've made this salsa as an appetizer for many dinner parties over the years, and it has always been a huge hit-so much so that there is rarely any left by the time I leave the kitchen to serve myself! Great with corn chips or served on top of fish.
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