Bring a large sauce pot of water to a boil. Cook the fettuccine according to package directions (about 7 minutes for al dente pasta). Reserve 1 cup of past water before draining and setting aside.
While the pasta cooks, melt the butter in a medium sauce pot over medium heat. Continue cooking once melted for 5 to 7 minutes, stirring constantly as the butter browns. It will foam and sizzle around the edges and turn a light brown color when it is done browning.
Add the garlic to the butter and cook about 30 seconds, stirring regularly. Whisk the flour into the evaporated milk. Add the evaporated milk and flour mixture and 2% milk to the pot, whisking as you add it to work out the flour lumps. Add the pumpkin puree and whisk until smooth. Season with sage, nutmeg, cinnamon, salt and pepper.
Simmer the sauce until it begins to thicken, whisking regularly to prevent burning. Once the sauce reaches the desired thickness, remove from heat. Stir in the Parmesan cheese and season with additional salt and pepper, if desired. If the sauce is thicker than desired, thin it with the reserved pasta water. We usually add about ¼ cup of pasta water to thin our sauce down to our desired consistency.
Pour the Alfredo sauce over the cooked and drained pasta. Stir well to incorporate. Serve and enjoy!