Melt 6 tablespoons of the butter in a frying pan. Add the flour, stirring until smooth and browned. Add the beef stock from the roasting pan, stirring until the sauce comes to a boil. Add the wine, tomato sauce and salt. Mix well. Cover and cook over low heat 45 minutes.
Melt the remaining butter in another pan. Sauté the sliced mushrooms 5 minutes, stirring occasionally. Add the sherry and pepper to taste.
Heap mushrooms and their cooking liquid over the beef. Slice it and serve with a separate bowl of the gravy.