01 - Preheat the oven to 400°F. Line a baking sheet with parchment paper. Place the butternut squash halves cut side down on the sheet and roast for 35 to 40 minutes until tender. Allow to cool slightly.
02 - Scoop out the flesh and mash it until smooth. Measure 1 1/2 cups (approximately 340 g) of the purée and let it cool completely.
03 - In a large bowl, combine the cooled squash purée, egg, Parmesan, salt, and nutmeg if using. Gradually add flour, mixing gently until a soft, slightly sticky dough forms. Avoid overmixing.
04 - Lightly flour a clean surface. Divide the dough into four portions. Roll each into a rope about 3/4 inch (2 cm) thick and cut into 1-inch (2.5 cm) pieces. Optionally, roll each piece over the back of a fork to create ridges.
05 - Bring a large pot of salted water to a gentle boil. Add gnocchi in batches. When they float to the surface after 2 to 3 minutes, remove with a slotted spoon and set aside.
06 - In a large skillet, melt the butter over medium heat. Add the sage leaves and cook until the butter is golden and aromatic, about 2 to 3 minutes.
07 - Add the cooked gnocchi to the skillet and toss gently to coat and heat through for 1 to 2 minutes.
08 - Serve immediately, garnished with additional grated Parmesan and freshly ground black pepper.