1) Preheat the oven to 180C/Gas 4.
2) Peel potatoes and cut diagonally into 0.5cm thick slices.
3) In a saucepan combine 750ml of the single cream, garlic, freshly grated nutmeg and salt and pepper, to taste. Bring to a boil over moderate heat.
4) Butter a oval gratin dish, layer potatoes and cheese (reserving 25g of the cheese for the topping) and pour the cream mixture over the potatoes. Sprinkle the top of the gratin with the reserved cheese and dot with the butter. Transfer the gratin to the oven and bake for 30 to 40 minutes, or until bubbling and golden.
5) Remove from the oven and let it rest for 5 minutes before serving.