Melt the oil and butter in a large saucepan over a gentle heat. Add the onion and cover with a lid. Cook slowly for approx 10 mins or until the onions are soft and translucent.
Add the sugar and balsamic vinegar to the onions and stir well. Cover with the lid and continue to cook for a further 5 minutes.
Add the stock and boil gently uncovered for 5 minutes.
In a heatproof jug or bowl mix the corn starch/flour with the cold water to a thin paste. Pour a little of the hot gravy into the starch mixture and mix thoroughly. Pour the starch mixture back into the gravy, raise the heat to high and boil for 10 minutes or until the gravy is slightly thickened. Keep warm until ready to serve.