Halve the butternut squash and scoop out the seeds; score the flesh. Cut the top off the head of garlic to expose the cloves.
Arrange the squash and garlic on a baking sheet. Drizzle with olive oil, add a pinch of chili if you like, season with salt and pepper, and tuck the fresh sage underneath. Turn to coat.
Roast at about 180C (350F) for 45 minutes, until the squash is very tender.
Scoop the roasted squash flesh into a blender. Squeeze in the roasted garlic confit, add the chili, a generous spoonful of mascarpone, and a drizzle of olive oil, then blend smooth.
Cook the spaghetti in well-salted water until al dente, reserving some pasta water.
Loosen the sauce with a little pasta water and stir in the grated Parmesan. Toss with the al dente pasta and more pasta water, mixing vigorously until glossy.
Twirl into bowls and finish with extra sauce and Parmesan. Serve hot.
