Preheat the oven to 350°F and line a baking sheet with foil. Place the pumpkin, onion, and garlic on the baking sheet. Drizzle with olive oil and sprinkle with salt and pepper. Turn the pumpkin cut-side down and pierce a few holes in the skin with a fork. Cover with another sheet of foil and bake until the onion is soft and the pumpkin flesh is very tender, 35 minutes to 1 hour.
Add the sage to the pan during the last few minutes. Remove the pan from the oven, keeping everything covered, and let it continue to steam for another 10 to 15 minutes.
Pour the vegetable broth into a high-speed blender. Remove the skin and add the pumpkin flesh to the blender along with the onion, peeled garlic, sage, cashews, ½ teaspoon salt, and freshly ground black pepper. Blend until smooth and creamy. Add the 2 tablespoons olive oil and blend again. (If your sauce is not smooth and creamy, add a little more broth or olive oil and blend it longer).
Cook the pasta in a large pot of salted boiling water according to package directions or until al dente. Reserve ½ cup of the starchy pasta water before draining the pasta.
Drain the pasta and return it to the pot. Stir in half the sauce, adding more as needed to coat the pasta. Add the reserved pasta water, ¼ cup at a time, to thin the sauce and make it creamy. Add the remaining sauce as desired. Taste and season with more salt and pepper. Serve hot.