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1
Preheat oven to 400 degrees F (200 degrees C).
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2
Roast sweet potatoes in the preheated oven until tender, about 1 hour; set aside until cool enough to handle.
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3
Halve cooled sweet potatoes lengthwise and scoop the flesh from the peels. Discard potato peels. Mash the sweet potato flesh on a work surface and allow to rest for at least 30 minutes to cool and release moisture.
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4
Transfer mashed sweet potatoes to a bowl and mix with egg yolks, ricotta cheese, and 3/4 cup Parmigiano-Reggiano cheese. Stir in salt, black pepper, and nutmeg. Add flour gradually to the sweet potato mixture until the dough holds together. Start with 1/4 cup each semolina flour and all-purpose flour, adding more as you need to.
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5
Scoop dough by teaspoonful and roll into marble-size balls. Sprinkle a baking sheet generously with semolina flour and place gnudi onto the prepared baking sheet. Refrigerate at least 1 hour or overnight for best texture.
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6
Bring a large pot of salted water to a boil. Reduce heat to low and let the water simmer. Gently drop gnudi, about 12 at a time, into the simmering water and cook until they rise to the surface, about 4 minutes. Simmer for 4 more minutes and remove with a slotted spoon. Keep boiled gnudi warm while you finish cooking remaining batches.
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7
Melt butter in a large skillet over medium heat and cook whole sage leaves until they wilt and are beginning to brown, about 2 minutes; remove leaves. Retain butter in skillet. Stir chopped sage into the hot butter and cook until the butter begins to brown and give off a nutty fragrance, about 2 minutes. Gently mix the gnudi into the butter and sage until gnudi are coated. Garnish with whole sage leaves and sprinkle with 2 tablespoons Parmigiano-Reggiano cheese to serve.
