Peel and finely slice the onions. Heat the olive oil with the butter in a frying pan over a medium heat. Add the onions and fry for 3-4 minutes, until they are golden. Add the red wine and simmer until the volume is reduced by half.
Meanwhile, in a separate pan, warm the beef stock. When the onion is golden brown, add the beef stock and simmer to reduce again until the gravy is thickened then season to taste with salt and freshly ground black pepper.