Preheat oven to 425 degrees F, and line a large rimmed baking sheet with parchment or a silicone baking mat for easy clean-up.
Combine squash and onion on the baking sheet. Sprinkle with 1 tablespoon olive oil, rosemary, 1/2 teaspoon salt, and 6-8 cranks of fresh-ground black pepper. Toss to combine. Roast for 20-25 minutes, tossing halfway through cooking, until the squash is tender and starting to brown on the edges.
3-4 cups cubed butternut squash, 1/2 red onion, 2 Tablespoons olive oil, 1 Tablespoon chopped fresh rosemary, kosher salt and black pepper
When the squash is about halfway done, boil a large pot of well-salted water and cook pasta to al dente, according to package directions. Drain and set aside.
8 ounces pasta
Cook bacon, if needed, and crumble into small pieces.
2 pieces cooked bacon
While pasta cooks, place a large skillet or pot over medium heat. Add walnuts and toast in the dry skillet for 2-3 minutes, stirring frequently and taking care that they don’t start to blacken or burn. Once the walnuts are lightly toasted and fragrant, add butter and remaining 1 tablespoon olive oil to the pan.
1/4 cup chopped walnuts, 3 Tablespoons butter
When butter is melted, stir in the drained pasta, roasted squash, crumbled bacon, and Parmesan. Toss to combine. Garnish with additional sprigs of rosemary, if desired, and season with more salt and pepper to taste.
1/2 cup shredded or shaved Parmesan
Notes
Squash: For best results, use squash cut into cubes of approximately 1". This allows them to roast quickly and evenly. Feel free to substitute another variety of winter squash.
Onion: You can substitute 1-2 chopped medium shallots for a slightly different flavor, or use a white or yellow onion in a pinch.
"Sauce": This is not a very "saucy" pasta by its nature. If you prefer more of a sauce, reserve some of the pasta water just before draining, then mix it back into the skillet, just a few Tablespoons at a time, after adding the pasta and squash.