Chop 6 large onions in a food processor and heat with a small amount of oil in a stainless steel pan until they are soft and starting to colour.
Add 1 cup of balsamic vinegar and 1 cup of brown sugar.
Bring the mixture to the boil and then simmer gently for 1/2 an hour to an hour, until the onions have gone translucent and all the liquid has evaporated.
Pour the hot mixture straight into small (200g) sterilised jars, seal, and store somewhere cool and dark or in the fridge. The relish should keep for at least 6 months.
Relish should be left for 4-6 weeks to mature allowing all of the wonderful flavours to develop and mix.
You can add variations according to your personal taste, such as wholegrain mustard.