Let’s dive into the step-by-step process to bring this comforting dish to life:

1. Preheat your oven to 400°F (200°C). Spread the cubed butternut squash on a baking sheet, drizzle with olive oil, and season with salt and pepper. Roast for about 25-30 minutes or until tender and slightly caramelized.
2. While the squash is roasting, cook the pasta in a large pot of salted boiling water according to package instructions until al dente. Reserve 1 cup of pasta water, then drain the pasta and set it aside.
3. In a large skillet, melt the butter over medium heat. Add the chopped onion and sauté until translucent, about 3-4 minutes. Stir in the minced garlic and cook for another minute.
4. Add the roasted butternut squash to the skillet, along with the chopped sage and a pinch of nutmeg. Toss everything together, allowing the flavors to meld for about 2 minutes.
5. Pour in the heavy cream and bring to a gentle simmer. Use a masher or fork to slightly mash some of the squash, creating a creamy sauce. If the sauce is too thick, add some reserved pasta water to reach your desired consistency.
6. Add the cooked pasta to the skillet and toss to coat. Stir in the Parmesan cheese and season with salt, pepper, and chili flakes to taste.
7. Serve hot, garnished with fresh parsley and additional Parmesan if desired.