Mashed potato is just as good with mashed chestnuts as it is in the classic combination of mashed potato and parsnip or celeriac. In all cases, the potato attenuates the sweetness and improves the texture of the accompaniment. This gratin auvergnat has the added attraction of a crisp, golden crust of Cantal cheese. It can be eaten as a first course or accompany boudin or roast meat.