01 - Preheat oven to 400°F. Line a baking sheet with parchment paper. Toss the cubed butternut squash with 1 tablespoon olive oil, salt, and black pepper. Arrange on the prepared baking sheet and roast for 25 to 30 minutes until tender and caramelized.
02 - Bring a large pot of salted water to a rolling boil. Add the pasta and cook according to package instructions until al dente. Reserve 1 cup of the pasta water before draining.
03 - Heat the remaining 1 tablespoon olive oil in a large skillet over medium heat. Add diced onion and sauté for 3 to 4 minutes until translucent. Stir in the minced garlic and chopped sage, and cook for 1 to 2 minutes until aromatic.
04 - Transfer the roasted squash to the skillet. Add ground nutmeg and red pepper flakes if desired. Pour in the heavy cream and half of the reserved pasta water. Simmer for 3 to 4 minutes, stirring occasionally.
05 - Using a blender or immersion blender, process the mixture until smooth and creamy. Adjust consistency by adding more reserved pasta water if necessary.
06 - Return the squash sauce to the skillet over low heat. Add the cooked pasta, unsalted butter, and Parmesan cheese. Toss until the pasta is thoroughly coated, adding additional pasta water as needed. Season with salt and black pepper to taste.
07 - Serve immediately, garnished with extra Parmesan, chopped sage, and toasted pumpkin seeds if desired.