Cook the fettuccine according to package directions until al dente. Reserve 1 cup of pasta water before draining.
While pasta cooks, melt butter in a large skillet over medium heat. Add minced garlic and sauté for 1-2 minutes until fragrant.
Stir in the pumpkin puree and heavy cream, whisking until smooth and well combined.
Add Parmesan cheese, nutmeg, salt, and pepper. Stir continuously until the cheese melts and the sauce is creamy, about 3-4 minutes.
Add the cooked pasta to the sauce and toss to coat. If the sauce is too thick, add reserved pasta water a little at a time until you reach desired consistency.
Serve immediately, garnished with fresh sage leaves and extra Parmesan if desired.