Step 1

Roast the sweet potatoes. Heat the oven to 400° F. Place the sweet potatoes on a baking sheet and prick a few holes in them with a fork. Bake until completely tender, 45 to 60 minutes.

Step 2

Cool and drain the sweet potatoes. Slice them open with a knife and let them cool briefly. Line a colander with a double layer of cheesecloth or paper towels and scoop the flesh into it. Squeeze or pat out as much liquid as you can. This step will rid the sweet potatoes of excess moisture and is especially important if you’re using moist, orange-fleshed potatoes. Let cool until barely warm.

Step 3

Rice or mash the sweet potatoes. Sprinkle 1/2 cup of the 2 cups flour on a clean work surface. Measure out 2 full cups (about 1 pound) of the sweet potato flesh and feed it through a potato ricer onto the floured work surface. Or use a potato masher or a fork to mash the potatoes in a bowl and turn them out onto the work surface (reserve any leftover sweet potato for another use).

Step 4

Make the gnocchi dough. Sprinkle all but 1/4 cup of the remaining flour around the potato mound. Make a well in the mound and add the egg and salt. With a fork, whisk the egg and salt together and begin to incorporate the potatoes and flour. Use a dough scraper to “chop” through the dough to help bring the ingredients together without overmixing. Sprinkle the remaining 1/4 cup flour over the dough mass and very lightly knead and pat the mixture until it forms a soft, shaggy ball. It should feel slightly tacky, maybe even a bit sticky. If the dough is too sticky to work, sprinkle in a little more flour, but be careful not to add too much, or to over-knead, as the dough will only get stickier as it continues to absorb flour. Cover the dough with a clean kitchen towel.

Step 5

Shape the gnocchi. Line two rimmed baking sheets with clean towels and lightly dust with flour. Have ready a fork or wooden gnocchi board for making grooves in the gnocchi. Sprinkle a small amount of flour on the work surface. Slice off a tangerine-size piece of dough and gently roll it out into a rope about 3/4-inch in diameter. Use the dough scraper or a knife to cut the rope into 1-inch nuggets. Sprinkle them lightly with flour. Roll the pieces of dough down the curved tines of the fork or down the length of the gnocchi board to create an indentation on one side and grooves on the other. Set the gnocchi on a prepared baking sheet, taking care to keep them separated. Continue to cut and shape the remaining dough, setting the gnocchi on the baking sheets as you work. You should end up with about 175 to 200 gnocchi.

Step 6

Let the gnocchi dry. Let the trays sit uncovered for about an hour; this will help to form a “skin” on the surface of the gnocchi, which will keep them from sticking when cooked. If not cooking within a couple of hours, place the baking sheets in the refrigerator or in a cold spot. They will keep for several hours. Cook them directly from the refrigerator.

Step 7

Boil a pot of water and warm a serving bowl. While the gnocchi are resting, bring a large pot of water to a boil over medium-high heat, and add a tablespoon of salt. Place a large serving bowl in the oven and turn the oven on to its lowest setting to warm the bowl.

Step 8

Make the sauce. In a large skillet, heat 4 tablespoons olive oil and 2 tablespoons butter over medium-low heat. When the butter is melted and begins to foam, add the garlic and cook for about 3 minutes, pressing down on the cloves to infuse the oil and butter. Don’t let the garlic brown. When you can smell the garlic’s aroma, remove the cloves and stir in the sage leaves, Cook on medium-low until wilted and just starting to crisp—about 3 minutes. Turn off the heat and cover the pan.

Step 9

Cook and serve. When the water is boiling, carefully drop in the gnocchi. Cook in batches to avoid overcrowding the pot. As the water returns to a boil the gnocchi will start floating to the surface. Let them boil gently for about 3 minutes, just enough to make them tender, but still the slightest bit firm, with no taste of raw flour (taste one to make sure). Use a skimmer to transfer the gnocchi to the warmed serving bowl. Spoon half the sauce on top, adding a little cooking water if needed to loosen the sauce. Toss gently and slide the bowl back into the oven. Cook the remaining gnocchi and transfer them to the warmed bowl. Spoon the rest of the sauce on top, adding more cooking water if needed to coat the gnocchi. Sprinkle Parmigiano cheese on top and serve.