Creamy Gratin of Celeriac, Parsnip, and Potato

Autumnal roots, sweet and sturdy, distinguish this useful dish—which, although creamy, does not contain cream. It is easily varied: For a milder taste, use half potatoes and half celeriac; for a more aromatic dish, add shallots and tarragon; for sweetness, include sweetpotatoes; or, for a meaty dish, add cubed smoked turkey or ham. The dish can be made ahead and reheated.