Heat the oil in a large frying pan over a low heat. Add the onions and a good pinch of salt and cook very slowly for 15-20 minutes, stirring them 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 the onions are softened and tinged golden, add the sugar and balsamic vinegar, this will start the caramelisation process. Cook the onions over low heat for a further 5-10 minutes, stirring occaisionally, until sticky and caramelised.