Toss butternut squash, onion, salt and olive oil together and spread in a single layer on a large rimmed sheet pan. Prick the sausages with a fork, then add to the sheet pan. Slice off the top of the head of garlic, wrap in foil, and add to the sheet pan. Roast for 30-35 min., turning vegetables and sausages once, until browned. Reduce the oven temperature to 375° F.
Cook the pasta in salted, boiling water until al dente, about 10 min. Drain well, reserving 1 cup of the pasta water.
Combine cooked pasta, sun-dried tomatoes, walnuts, sage, creme fraiche and reserved pasta water in a large bowl. Add roasted vegetables. Unwrap garlic and squeeze the roasted cloves into the mixture. Halve sausages lengthwise, then chop crosswise into chunky pieces. Add 3/4 cup of the mozzarella cheese to the pasta mixture. Gently mix all of the ingredients together.
Spray a 12 in. baking oval with non-stick cooking spray and pour in the pasta mixture. Top with remaining 1/4 cup mozzarella and parmesan. Pasta can be refrigerated, covered, at this point for up to 2 days.
Bake 15-20 min., until bubbly and lightly browned on top. If baking from the refrigerator, bake covered for 15-20 min., then uncover and bake an additional 15-20 min.