In a medium saucepan, combine chopped plums, apple cider vinegar, brown sugar, soy sauce, minced garlic, grated ginger, and red pepper flakes. Stir well and bring to vigorous boil for about 5 minutes.

Reduce heat to low and simmer for about 25 minutes, stirring occasionally, until plums are soft and sauce thickens.

Transfer the mixture into a blender or use an immersion blender to puree until smooth.

Return blended sauce to saucepan, bring to gentle boil. Ladle into sterilized jars, leaving 1/2 inch headspace.

Let jars cool at room temperature for 24 hours, then check seals and store in a cool, dark place.