First, peel the potatoes and slice them very, very thinly (a wooden mandolin is excellent for this operation, if you have one), then plunge the potato slices into a bowl of cold water and swill them round and round to get rid of some of the starch.Now dry them very thoroughly in a clean tea cloth. Then in the gratin dish arrange a layer of potato slices, a sprinkling of crushed garlic, pepper and salt and then another layer of potatoes and seasoning. Now mix the cream and milk together, pour it over the potatoes, sprinkle with a little freshly grated nutmeg, then add the butter in flecks over the surface and bake on the highest shelf in the oven for 1½ hours.For Perfect Mashed Potatoes, watch Delia's Cookery School Video on this page