Peel the potatoes and slice thinly on a mandolin or in a food processor. Drop the slices into cold water. Drain and then dry them thoroughly on absorbent paper. Season the potato slices with freshly grated nutmeg, salt and freshly ground black pepper to taste.

Heat 25 g (1 oz) of the butter in an 18 cm (7 in) frying-pan. Using half the potatoes, make 2 layers of potato slices in the pan, pressing them down into a 'cake' with a fish slice.

Cover the frying-pan with a lid and cook the potato cake for 2-3 minutes, or until the underside is browned. Reverse the potato cake onto the lid, return the cake to the pan and continue to cook for 2-3 minutes until the underside is done. Transfer to a heatproof plate and keep warm. Add another 25 g (1 oz) butter to the pan and repeat the process to make a second potato cake.

Heat the grill to high. Meanwhile, skin the kidneys, cut in half and remove the core, season with salt and freshly ground black pepper. Place the kidneys cut-side down on the oiled grill rack and brush with half the melted butter. Cook the kidneys 7.5 cm/3 in from the heat for 2 minutes. Turn the kidneys over with a fish slice, brush with the remaining melted butter and cook for a further 1 - 2 minutes. Keep the kidneys warm.

To serve, sprinkle each potato cake with 30 - 45 ml (2 - 3 tbsp) freshly grated Gruyere cheese and cook under the grill, 7.5 cm (3 in) from the heat, for 1 - 2 minutes, or until the cheese is just melted. Serve immediately, accompanying each cake with 2 grilled kidneys, and garnished with sprigs of parsley.