Make the pastry in the normal way by rubbing the fat into the flour and adding the potato purée. Bind with the water to make a smooth dough and put into the fridge for a half-hour.
For the pie, parboil or steam the potatoes for 5 minutes or until just tender. Blanch the onions for 4 minutes. Drain and arrange in a 9-inch pie dish with the potatoes, cheese, parsley, garlic and salt and pepper. Mix the nutmeg with the cream and pour over the whole.
Roll out the pastry and cover the pie. Brush with the beaten egg yolk. Bake at