Michael Harlan Turkell
Reprinted by arrangement with Avery Books, a member of Penguin Group (USA) LLC, A Penguin Random House Company. Copyright © Nancy Harmon Jenkins and Sara Jenkins, 2015.
Set the oven on 425 degrees F.
Prick the sweet potatoes all over with a fork. Place in the hot oven and bake until they are falling-apart tender, about 50 minutes. While the potatoes are still very hot, remove the peels and discard them. Mash the hot potatoes or put them through a food mill into a bowl. You should have about 4 cups pureed sweet potato.
Stir in the flour and mix well, beating with a wooden spoon. Break off a small walnut sized piece of dough and roll it into a ball. It should hold together well, with a firm but malleable texture. If the dough seems too loose, add a little more flour, but be careful not to add too much, since it will make the gnocchi heavy.
Dust a clean work surface or a pasta board with a bit more flour and spread the potato mixture out. Knead it, adding a little more flour, if necessary, just until the dough comes together, then cover the mound of dough with a clean kitchen towel and leave it to rest for about 10 minutes.
Bring about 4 quarts salted water to a rolling boil. On a counter next to the stove, fill a large bowl with ice cubes and water to be ready to receive the gnocchi once they are cooked.
While the water is heating, dust the work surface once more with a little flour. Break off a fistful of dough, leaving the rest under the towel. Using the palms of your hand, roll the dough into a rope, about 1 inch in diameter. Cut the rope into gnocchi, each piece about 1/ 3 inch long.
Holding your hand flat with the fingers together, briskly roll each gnocco on the board, back and forth under your fingers, to form a little dowel-shape piece, then roll through a little flour to dust lightly. Transfer to a lightly floured baking sheet. Repeat with the remaining dough, until all of the gnocchi are rolled.
Heat 4 tablespoons of the butter and ¼ cup of the oil in a small skillet, and as the butter begins to foam, add the sage leaves. Fry until the butter is nut-brown and the sage leaves are crisp. Set aside but keep warm.
Adjust the heat under the water so that the water boils gently but not fiercely. Gently drop the gnocchi, a few at a time, into the bubbling water. They will float to the surface when they are done. As they finish cooking, use a slotted spoon to skim them off and transfer to the ice-water bath, just dipping and then removing them before transferring them to a deep plate.
Have ready a warm serving bowl.
When all the gnocchi are poached, combine the remaining 4 tablespoons butter and ¼ cup oil in a large skillet and add the gnocchi, in batches. Toast them briefly, just long enough to heat them up once more. Add the first batch to the warm bowl.
Spoon a little of the sage and brown butter over the top, then a little of the grated Parmigiano-Reggiano, and then continue, layer by layer, until all the gnocchi are cooked. Spoon the last sage and brown butter mixture over the top, and dust with the remaining cheese. Add plenty of cracked pepper and serve immediately.