In a large frying pan heat the oil over a high heat.
Add the sliced onions, reduce the heat to low and fry the onions slowly. Cover the pan and cook for approximately 15 minutes or until the onions are soft.
Remove the lid. Turn the heat back up to high and stir continuously. Pour in the vinegar and Madeira and cook for a further minute to deglaze the pan.
Melt the beef