Preheat the oven to 200C/gas 6/fan 180C. Peel and deseed the squash, then cut it into mouthful-sized chunks and tip into a roasting tin. Peel the onions, leaving the root intact. Then cut each, through the root, into eight wedges and put them in the tin.
Add the garlic to the tin with 2 tablespoons of olive oil. Sprinkle generously with salt and pepper, then toss everything together with your hands until all the ingredients are glistening. Roast for 30-35 minutes, until everything has browned nicely.
Meanwhile, cook the pasta in a large pan of salted water following the timings given on the packet. Drain the pasta, reserving about 4 tbsp of the cooking water. Remove the tin from the oven and add the cooking water and crème fraîche, stirring everything together lightly. Tip in the pasta and toss well. Serve with a sprinkling of freshly grated parmesan.