Finely chop the onion and chilli.
Wash the plums, cut them in half, and remove the stones.
Crush the coriander seeds.
Put the oil in a heavy-based pan and sauté the onion, chilli and rosemary for about 8-10 minutes over low heat.
When the onion is soft, add the rest of the ingredients (except the fish sauce), then and bring to the boil. Put a lid on, turn down the heat and cook for about an hour until it reduces to a thick sauce. Add the fish/soy sauce and cook for another 20-30 minutes, until you get a jammy consistency.
Let it cool, remove the cardamom shells & cinnamon sticks, then spoon the chutney into sterilised jars.Seal the jars well.
Store in a cool place. Once opened, store in the refrigerator.