Preheat oven to 400 degrees. Peel, seed and cut squash into 1 inch cubes. Put 2 Tbsp. olive oil in a large jelly roll or roasting pan. Spread out squash and onion in a single layer in pan. Sprinkle with 1 ½ tsp. coarse salt and toss in pan. Roast for 10 minutes. Stir. Then roast for 10 more until squash is soft.
In meantime, in a large skillet over medium heat, stir garlic and bell pepper with 2 tbsp. olive oil for 1 minute. Add sage and walnuts for another minute.
Add squash mixture to skillet.
Cook pasta in boiling, salted water for 3 minutes. Before draining, ladle ¾ cup cooking liquid into skillet with squash. After testing for doneness, drain and toss in skillet with parmesan. Salt and pepper to taste.
Garnish with more parmesan and parsley. Serve.