Bring a large pot of salted water to a boil. Add the 8 oz of fettuccine and cook according to the package instructions until al dente, about 10-12 minutes. Drain, reserving 1/2 cup pasta water, and set aside.
While the pasta cooks, heat 2 tablespoons olive oil in a large skillet over medium heat. Add chopped onion and sauté until translucent and fragrant, about 3-4 minutes. Stir in minced garlic and cook 1 more minute without browning.
Lower heat to medium-low. Add 1 cup canned pumpkin puree and stir to combine. Slowly pour in 1 cup heavy cream, stirring continuously to create a smooth sauce. Simmer gently for 3-4 minutes.
Add 1 tablespoon fresh chopped sage, then stir in 1/2 cup grated Parmesan cheese until melted. Season with salt and freshly ground black pepper to taste.
Add cooked fettuccine to the skillet and toss to combine. If sauce is too thick, loosen with reserved pasta water. For extra protein, toss in cooked chicken or turkey.
Divide pasta among plates. Garnish with additional Parmesan cheese and fresh sage leaves if desired. Serve immediately.
