Add the onions and a good pinch of salt and cook very slowly for 15-20 minutes stirring occasionally to prevent them from sticking.
Don’t turn up the heat to cook faster, slowly cooking releases more of their flavour
When onions are softened and slightly browned, add sugar and balsamic vinegar – this will start the caramelisation process.
Continue to cook onions over low heat for a further 5-10 minutes, stirring occasionally, until sticky and caramelised.
Use immediately, or store in the fridge, in an air tight container, and use when required.