Put the walnuts in a food processor or blender with the garlic, anchovies and their oil, horseradish, parsley, 1 tablespoon of the oil and plenty of black pepper and blend to a thick paste, scraping the mixture down from the sides of the bowl.
Untie the beef and open it out slightly. If there is already a split through the flesh, make the cut deeper so that it will take the stuffing. If it is a perfectly rounded piece of beef, make a deep cut so that you can pack in the stuffing. Once the stuffing is in place, reshape the meat into a roll. Tie with string, securing at 2.5 cm (1 inch) intervals. Pat the meat dry with kitchen paper and season with salt and pepper.
Heat the remaining oil in a flameproof casserole and fry the meat on all sides to brown. Drain to a plate.
Add the onion and celery to the pan and fry gently for 5 minutes. Return the meat to the pan and pour the wine and stock over it. Add the carrots and turnips. Bring just to the boil, cover with a lid and place in a preheated oven, 160°C (325°F), Gas Mark 3. Cook for 30 minutes.
Tuck the potatoes around the beef and sprinkle with salt. Return to the oven for a further 40 minutes until the potatoes are tender. Stir in the beans and return to the oven for 20 minutes until the beans have softened. Leave to rest for 15 minutes before carving the meat.