Heat oil in a large frypan over low heat. Add the onions and a good pinch of salt and cook very slowly for 25 minutes, stirring occasionally to prevent them from catching. Don't be tempted to turn the heat up, as you don't want the onions to burn.
When onions are softened and tinged golden, add the sugar and balsamic - this will start the caramelisation process. Cook onion over low heat for a further 5-10 minutes, stirring occasionally, until sticky and caramelised. Use immediately, or store in the fridge, in a sterilised jar or plastic container, and use when required.ths.
