Ingredients
3 tablespoons unsalted butter, softened
4 ounces Parmigiano-Reggiano, finely grated (1 cup)
Kosher salt and freshly ground pepper, plus 7 whole black peppercorns
5 cups half-and-half
32 fresh sage leaves (from 1 bunch)
4 cloves garlic, smashed
3 pounds sweet potatoes, such as Jewel, Garnet, or Beauregard, unpeeled
Extra-virgin olive oil, for drizzling
Preparation
Preheat oven to 375°F. Brush butter evenly over bottom and sides of a 9-by-12-inch oval baking dish, 8-by-10-inch rectangular baking dish, or other shallow 2-to-2 1/2-quart dish. Sprinkle 1/2 cup cheese evenly over butter; season with ground pepper.
Combine half-and-half, 20 sage leaves, garlic, and peppercorns in a large saucepan; season with 1 tablespoon salt. Bring to a boil, then reduce heat to medium and simmer until reduced by half, 15 to 20 minutes. Strain, discarding solids; season to taste.
Meanwhile, slice sweet potatoes into 1/8-inch-thick rounds. Snugly arrange slices vertically in prepared baking dish. Tuck remaining 12 sage leaves between potatoes; carefully pour half-and-half mixture over top. Sprinkle top with remaining 1/2 cup cheese and lightly drizzle with oil.
Roast until potatoes are tender and top is golden brown, 55 to 65 minutes. (If top is browning too quickly, tent with foil.) Let cool at least 15 minutes before serving.
