Sweet Potato Gnocchi with Fried Sage and Shaved Chestnuts

Name: Sweet Potato Gnocchi with Fried Sage and Shaved Chestnuts
Ingredients: 1 1/4 pounds russet (baking potatoes); 1 (3/4-pound) sweet potato; 1 large egg; 1/2 teaspoon grated nutmeg; 1/3 cup grated Parmigiano-Reggiano plus more for serving; 1 1/2 to 2 cups all-purpose flour plus more for dusting; 1/3 cup extra-virgin olive oil; 1 cup sage leaves (from 1 bunch); 1/3 cup bottled roasted chestnuts, very thinly sliced with an adjustable-blade slicer or a sharp vegetable peeler; 2 tablespoons unsalted butter; Equipment: a potato ricer or a food mill fitted with fine disk