Remove any fat from around the kidneys. Snip out the core using a pair of scissors and remove the skin. Place in a basin, cover with cold water and add the vinegar. Leave to soak for 10–15 minutes. Drain, pat dry and slice thickly. Add to the hot butter in a frying pan and fry gently for 5 minutes; do not overcook. Then add a good seasoning of salt and pepper and sprinkle over the flour. Stir to blend, then add the cream and mustard to taste. Bring up to the boil, stirring until thickened, then draw the pan off the heat. Serve with slices of hot buttered toast.