01 - Preheat oven to 400°F. Line a baking sheet with parchment paper. Place squash halves cut side down and roast for 35 to 40 minutes until soft. Allow to cool slightly.
02 - Scoop out flesh and mash until smooth. Measure 1 1/2 cups of purée and cool completely before use.
03 - In a large bowl, combine cooled purée, egg, Parmesan, salt, and nutmeg if using. Gradually add flour, mixing gently until a soft, slightly sticky dough forms without overworking.
04 - Lightly flour a surface. Divide dough into 4 portions. Roll each into a 3/4-inch diameter rope. Cut into 1-inch pieces and optionally roll over the back of a fork to create ridges.
05 - Bring a large pot of salted water to a gentle boil. Cook gnocchi in batches until they float, about 2 to 3 minutes. Remove with a slotted spoon and set aside.
06 - In a skillet, melt butter over medium heat. Add sage leaves and cook until butter is fragrant and golden, about 2 to 3 minutes.
07 - Add cooked gnocchi to the skillet and toss gently to coat and heat through, 1 to 2 minutes. Serve immediately garnished with extra Parmesan and freshly ground black pepper.