First cut the kidneys in half lengthways and remove the sinews. Put in a bowl with the milk and leave to soak for at least one hour.
Meanwhile heat the oil and the butter in a frying pan. Slice the potatoes into circles of an even thickness. Pan fry them in the oil and butter slowly on each side. They will be ready when they are golden and are tender. Remove from the oil and strain on some kitchen paper and keep warm.
Next make the sauce. Place wine, chicken stock and cream in a saucepan and bring to the boil. Reduce the heat slightly and continue to cook until the sauce reduces and thickens. Stir in the wholegrain mustard and continue to reduce until it has a sauce-like consistency and then add the butter. Keep warm.
Remove the kidneys from the milk and pat dry with kitchen paper. Season the flour with salt and pepper and dust the kidneys with it. Heat the butter in a frying pan over a medium to high heat and then fry them so that they are caramelised on the outside, but still pink in the middle.
To serve, place 3 pieces of potato on each plate. Place a piece of kidney on each and drizzle with the mustard sauce and garnish with the rocket.