Sauté the onions, chilli and rosemary in the oil for 10 minutes over a medium to high heat. Add all the remaining ingredients except the fish sauce and bring slowly to the boil, stirring occasionally to prevent the relish sticking to the bottom of the pan. Turn the heat down and simmer for 1 hour, then add the fish sauce. Continue to cook for about another 30 minutes until the relish is quite thick. Be sure to stir every 10 minutes or so as it thickens or it may catch on the bottom and burn.
When it is ready, spoon into hot, clean jars (rinse them out with hot water beforehand) and seal while still hot. Leave to cool and store in the fridge for at least a week before using. Stored this way the relish will keep for up to six months.
© 1997 Peter Gordon. All rights reserved.