While the Yorkshires cook, put the beef roasting tin over a medium heat and sprinkle the flour over the onions. Cook, stirring, for a minute or two to cook out the flour then gradually add the red wine and stock, whisking to prevent lumps. Bring to the boil and simmer for 5 minutes until thickened. Add any juices that collect in the platter under the rested beef and stir well. Strain through a sieve into a jug and keep warm. Carve the beef into thick slices and serve with the gravy, horseradish, Yorkshires and watercress or steamed green veg.