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1
Preheat oven to 450u00b0F with rack in middle.
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2
Pierce russet and sweet potatoes in several places with a fork, then bake in a 4-sided sheet pan until just tender, 45 minutes to 1 hour.
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3
Cool potatoes slightly, then peel and force through ricer into sheet pan, spreading in an even layer. Cool potatoes completely.
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4
Lightly flour 2 or 3 large baking sheets or line with parchment paper.
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5
Beat together egg, nutmeg, 1 teaspoon salt, and 1/2 teaspoon pepper in a small bowl.
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6
Gather potatoes into a mound in sheet pan, using a pastry scraper if you have one, and form a well in center.
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7
Pour egg mixture into well, then knead into potatoes. Knead in cheese and 1 1/2 cups flour, then knead, adding more flour as necessary, until mixture forms a smooth but slightly sticky dough. Dust top lightly with some of flour.
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8
Cut dough into 6 pieces. Form 1 piece of dough into a 1/2-inch-thick rope on a lightly floured surface. Cut rope into 1/2-inch pieces. Gently roll each piece into a ball and lightly dust with flour.
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9
Repeat with remaining 5 pieces of dough.
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10
Turn a fork over and hold at a 45-degree angle, with tips of tines touching work surface. Working with 1 at a time, roll gnocchi down fork tines, pressing with your thumb, to make ridges on 1 side. Transfer gnocchi as formed to baking sheets.
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11
Heat oil in a 12-inch heavy skillet over medium heat until it shimmers. Fry sage leaves in 3 batches, stirring, until they turn just a shade lighter and crisp (they will continue to crisp as they cool), about 30 seconds per batch. Transfer to paper towels to drain. Season lightly with salt.
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12
Fry chestnuts in 3 batches, stirring, until golden and crisp, about 30 seconds per batch. Transfer to paper towels to drain. Season lightly with salt. Reserve oil in skillet.
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13
Add butter to oil in skillet with 1/2 teaspoon salt and cook until golden-brown, 1 to 2 minutes. Remove from heat.
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14
Add half of gnocchi to a pasta pot of well-salted boiling water and stir. Cook until they float to surface, about 3 minutes. Transfer with a slotted spoon to skillet with butter sauce. Cook remaining gnocchi in same manner, transferring to skillet as cooked.
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15
Heat gnocchi in skillet over medium heat, stirring to coat.
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16
Serve sprinkled with fried sage and chestnuts and grated cheese.
