Butternut Squash Penne

butternut squash, about 2lb (900g), peeled and seeded

red onion, cut into eighths

olive oil, plus more for serving

balsamic vinegar

Salt and freshly ground black pepper

garlic cloves, minced

dried penne

shredded Gruyere

creme fraiche

chopped sage

Freshly grated Parmesan, to serve