I love cooking this dish because it makes the kitchen smell so amazing and festive. It takes a while to cook, but it is so simple to prepare and can be made in advance.
In a large saucepan over a medium heat, melt the butter and fry the onion for 5 minutes to soften. Add the sugar and balsamic vinegar and cook for a further 10 minutes until the onion is sticky and caramelised.