3 pounds russet potatoes, peeled and sliced crosswise 1/4 inch thick
5 cups heavy cream
Salt and freshly ground white pepper
Preheat the oven to 250° and butter a 13-by-9-inch baking dish. In a large pot, combine the potatoes and cream and season with salt and pepper. Bring to a boil, then press a round of parchment or wax paper on the potatoes. Simmer over moderately low heat until the potatoes are tender, 15 to 20 minutes.
Using a slotted spoon, transfer the potatoes to the baking dish in layers. Season the potatoes with salt and pepper as you go. Return the pot with the cream to high heat and boil, stirring constantly, until the cream is reduced to 2 1/2 cups, 6 to 7 minutes. Pour the cream over the potatoes while shaking the baking dish.
Bake the gratin for 1 1/2 hours, or until golden on top. Blot any excess fat and let stand 15 minutes before cutting.
The gratin can be prepared through Step 2 and refrigerated overnight.