1. Bring a large pot of salted water to a rolling boil. Add the fettuccine and cook according to package directions until al dente. Before draining, reserve about 1 cup of the starchy pasta water. Drain the pasta and set aside.

2. While the pasta is cooking, melt the butter in a large skillet or Dutch oven over medium heat. Add the finely chopped yellow onion and cook until softened and translucent, about 5-7 minutes. Stir in the minced garlic and cook for another minute until fragrant, being careful not to let it brown.

3. Pour the pumpkin puree into the skillet with the onion and garlic. Stir well to combine. Gradually whisk in the heavy cream until the mixture is smooth and lump-free.

4. Add the dried sage and ground nutmeg to the pumpkin cream mixture. Season generously with salt and freshly ground black pepper. Bring the sauce to a gentle simmer, then reduce the heat to low and let it cook for 5 minutes, allowing the flavors to meld and the sauce to thicken slightly.

5. Remove the skillet from the heat. Stir in the grated Parmesan cheese until it is fully melted and incorporated into the sauce, creating a smooth and creamy consistency.

6. Add the cooked and drained fettuccine directly into the skillet with the pumpkin Alfredo sauce. Toss gently to coat all the pasta evenly. If the sauce seems too thick, add a splash or two of the reserved pasta water until it reaches your desired consistency.

7. Divide the pasta among serving bowls. Garnish with extra grated Parmesan cheese and fresh chopped parsley, if desired. Serve hot and enjoy!