For the perfect gratin, use a (fairly) firm potato, like Nicola, Charlotte, Belle de Fontenay and Gourmandine.
Preheat the oven to 160°C.
Cut the potatoes in slices about the thickness of a Euro, and wash and dry the slices. Use a clove of garlic cut in half to rub along the inside of a shallow casserole dish and repeat this with butter. Overlap the slices layer by layer in the dish, sprinkling each layer with pepper, salt and a touch of nutmeg. Pour cream or a mixture of cream and full milk over this until just covered and place some flakes of butter on top.
Put the dish in the oven for an hour and a half at 160°C. For the last 15 minutes, turn the oven up to 180°C to 200°C.
Tip: If an hour and a half is too long, first half-cook the potatoes in a milk-cream mixture and then halve the initial cooking time.