Add a drizzle of oil to a large pan over a low heat. Add the onions and a pinch of salt. Cook slowly, stirring occasionally, until caramelised; this will take around 30-40 minutes. Once mostly caramelised add the sugar and vinegar and cook for a further 5-10 minutes. If you want to skip this step use caramelised onion jam. Allow the onions to cool slightly.