12. For the red wine sauce, melt half of the butter in a frying pan and add the onions. Fry the onions until golden-brown, then add the stock and red wine. Bring to the boil, then reduce the heat to simmer the sauce until it has reduced by half and slightly thickened. Pass through a sieve into a clean pan.
13. Whisk in the remaining butter and season, to taste, with salt and freshly ground black pepper.