Preheat the oven to 375°. Drizzle the cut side of the butternut squash with olive oil and season with 1 teaspoon chili powder and some of the salt and pepper. Place the squash cut-side-down into a rimmed baking sheet. Roast until the squash is soft, about 20 to 30 minutes. Scoop out the squash from the skin. Place the squash into the bowl of a food processor (or use an immersion blender) to puree.
Prepare the sauce by melting the butter in a saucepan over medium heat. Cook the onion in the butter until soft, about 5 minutes.
Whisk in the flour and cook for 1 minute more. Whisk in the sherry until smooth.
Stir in half-and-half, and chicken stock. Continue to stir until the sauce thickens, about 1 to 2 minutes. Add the sage leaves, cover the pot, and reduce the heat to the lowest setting for 2 to 3 minutes so that the sage infuses the sauce.
Remove the sage leaves from the sauce. Stir in the butternut squash puree and Parmesan cheese. Season with 1 teaspoon chili powder, salt and pepper.
Cook the pasta, in salted, boiling water until al dente, just a couple of minutes. Drain the pasta (reserving some of the cooking liquid) and toss in the sauce. At this point if the sauce is thicker than you like you can add a ladle or two of pasta water to thin.
Garnish with chopped fresh parsley.