Preheat oven to 325°.
In a large bowl, mix the cream, crème fraiche, milk, nutmeg, salt and pepper.
Cut the garlic in half and rub the inside of a large baking dish with the garlic. Butter the baking dish so that it is well coated.
Line the bottom of the dish with a layer of the potatoes and sprinkle with some of the Gruyere. Dot the potatoes with a little butter. Repeat this: layer of potatoes covered with the Gruyere and dots of butter, until you use up all the potatoes. Pour cream mixture over the potatoes in the baking dish, making sure that they are almost completely submerged. Bake in the oven for 1 hour.