To make the caramelised onions, melt the vegan butter in a large pan and add the sliced onions, sea salt, black pepper and thyme. Stir well and cook over low heat for 40 minutes, stirring regularly, until soft and golden. Don’t let the onions stick to the pan or burn. After 40 minutes, add the sugar and balsamic vinegar and continue to cook on low for 25 minutes, until the onions are deeply brown and sticky. Let cool before using.