Traditionally piped through a pastry bag into rosettes to garnish a roast, this classic French potato mixture is here transformed into a rich casserole that can be prepared in advance and is still the perfect match for a prime rib roast.
Cooks' notes:
·Gratin can be prepared (but not baked) 1 day ahead and cooled completely, uncovered, then chilled, its surface covered with buttered wax paper and dish wrapped in plastic wrap. Bring to room temperature, about 1 hour, before baking.
·If you have only one oven, bake gratin while rib roast stands.
