- Heat the oil and butter in a large pan. Add the salted and peppered onions and fry until golden brown.
- Add the brown sugar, turn down the heat and continue frying for 15 minutes with the lid closed, until the onions are softer and a little more browned.
- Now add the sherry vinegar and red wine and simmer uncovered, stirring occasionally, until the liquid has thickened.
- The sauce can be served warm or cold. The taste is enhanced if you prepare the sauce a day in advance.