Heat the oil in a large saucepan and add the onions, peppers and celery. Fry gently for 10 minutes until soft but not browned, then add the salt and garlic.
Add all the remaining ingredients and bring to the boil. Reduce the heat and cook, uncovered, for 15 minutes, stirring occasionally.
Ladle into warm, dry jars, pressing the vegetables well down into the juices, then top with screw-top lids and leave to cool. This relish does not need time to mature but, if not immediately consumed, label unopened jars and store in a cool, dark place.
To serve, this relish makes a great accompaniment to homemade burgers.