Step 1

Heat oil in a large frypan over low heat. Add the onions and a good pinch of salt and cook very slowly for 15-20 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.

Step 2

When onions are softened and tinged golden, add 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.