In a large pot, bring salted water to a boil. Add the fettuccine and cook according to package instructions until al dente. Reserve 1/2 cup of pasta water, then drain the pasta and set aside.
In a large skillet, heat the olive oil over medium heat. Add the chopped onion and sauté until translucent, about 3-4 minutes. Then, add the minced garlic and cook for another minute until fragrant.
Stir in the pumpkin puree and heavy cream until well combined. Bring the mixture to a gentle simmer, then add the chopped sage, nutmeg, salt, and pepper. Let it cook for about 5 minutes, stirring occasionally.
Add the cooked fettuccine to the skillet with the pumpkin sauce. Toss the pasta to coat evenly, adding some reserved pasta water a little at a time until the desired consistency is reached.
Stir in the grated Parmesan cheese until melted and creamy. Adjust seasoning with more salt and pepper if needed.
Plate the pasta in bowls or on a large serving platter, and garnish with fresh sage leaves and additional Parmesan cheese if desired.
Notes
Adjust seasoning to taste and serve with extra Parmesan cheese.