Add mustard, coriander, and cardamom seeds to a skillet and toast, stirring frequently, until they pop, about 6-8 minutes. In a non-reactive pan on medium heat, add vinegar and brown sugar and allow sugar to dissolve. Add onions, ginger, and garlic and sauté for 10 minutes. Add plums, raisins, salt, pepper flakes, and toasted seeds and bring to a boil. Reduce heat and simmer 45-50 minutes, stirring consistently to prevent scorching. Allow to cool and place in a non reactive container with a lid.