Cook the pasta: Bring a large pot of salted water to a boil. Add the pasta and cook according to package instructions until al dente. Drain and set aside.
Sauté the aromatics: In a large skillet, heat the olive oil over medium heat. Add the chopped onion and cook until softened, about 3-4 minutes. Add the minced garlic and cook for another 1-2 minutes until fragrant.
Cook the mushrooms: Add the sliced mushrooms to the skillet and cook until they release their moisture and begin to brown, about 5-7 minutes.
Prepare the sauce: Reduce the heat to low and stir in the pumpkin puree, heavy cream, grated Parmesan cheese, dried thyme, and dried sage. Mix well until the sauce is smooth and heated through, about 3-4 minutes. Season with salt and pepper to taste.
Combine pasta and sauce: Add the cooked pasta to the skillet and toss to coat evenly with the sauce. Cook for another 1-2 minutes until everything is heated through.
Serve: Divide the pasta among four plates. Garnish with chopped fresh parsley and additional grated Parmesan cheese, if desired.