How to Make It
Combine whole plums and 1 cup water in a large saucepan. Cover and cook over medium until plums are soft, about 20 minutes. Remove from heat, and let cool 15 minutes. Remove and discard pits from plums. Place cooked plums in a food processor, and process until smooth. Transfer to a medium bowl.
Using a mortar and pestle, crush coriander seeds and fennel seeds with a pinch of salt. Add garlic, remaining 1/2 teaspoon salt, and, if desired, chopped chile; crush into seed mixture.
Add garlic mixture and fresh herbs to plum puree, and stir until well combined. Sauce may be stored in an airtight container in refrigerator up to 3 days.