Combine the oil, butter, onions and stock over a medium-high heat. Bring to a simmer and increase the heat. Stirring frequently, cook for 15 minutes or until all the liquid has evaporated.
Lower the heat and stir the honey and balsamic vinegar into the onion mixture. Then continue to cook for a further 15 minutes or until the onions are soft and caramelised.
Season well and cook for another minute before serving. Caramelised onions make a delicious topping for anything from hotdogs and sandwiches to tarts, salads and pasta dishes.