01 - Preheat the oven to 400°F. Prick sweet potatoes with a fork, place on a baking tray, and roast for 40–45 minutes until very tender. Let cool slightly, then peel and mash until smooth.
02 - Transfer mashed sweet potato to a large mixing bowl. Incorporate the egg, salt, pepper, and nutmeg (if using); mix thoroughly.
03 - Gradually blend in all-purpose flour, stirring until a soft, slightly sticky dough forms. Add just enough flour for the dough to hold together and remain workable.
04 - Dust a work surface with flour. Divide dough into 4 portions; roll each into a rope approximately ¾ inch thick. Slice into ¾ inch segments; optionally, roll each gnocchi over a fork to create ridges.
05 - Bring a large saucepan of salted water to a gentle boil. Working in batches, add gnocchi; once they float to the surface (about 2–3 minutes), lift out with a slotted spoon and drain.
06 - In a large skillet set over medium heat, melt unsalted butter. Add sage leaves and cook, swirling the pan, until butter is golden and nutty and sage is crisp, about 2–3 minutes.
07 - Add cooked gnocchi to the skillet, gently toss to coat in sage butter, and season with flaky sea salt and black pepper. Plate immediately and sprinkle with freshly grated Parmesan.