Cook the Pasta: Begin by bringing a large pot of salted water to a rolling boil. Once boiling, add the fettuccine and cook according to the package instructions until al dente, which usually takes around 8-10 minutes. Reserve 1 cup of the starchy pasta water, then drain the pasta in a colander and set it aside.
Sauté Aromatics: In a large, deep skillet, heat the olive oil over medium heat. Once hot, add the finely chopped onion. Sauté for about 5 minutes or until the onion becomes soft and translucent, stirring occasionally. Next, stir in the minced garlic and dried sage, cooking for an additional minute until the mixture becomes fragrant.
Create the Sauce: Lower the heat to its lowest setting and incorporate the canned pumpkin puree and heavy cream into the skillet. Stir the mixture well to ensure it is fully combined, then gently bring it to a simmer. Slowly add the grated Parmesan cheese, continuously stirring until the cheese has fully melted and the sauce has a creamy texture. If you find the sauce too thick, gradually mix in some of the reserved pasta water to achieve your desired consistency.
Combine Pasta and Sauce: Carefully add the cooked fettuccine to the skillet, using tongs to gently toss the pasta in the creamy pumpkin sauce until every strand is well coated. Taste and season with salt, black pepper, and crushed red pepper flakes if you want to add heat.
Serve: Once well mixed, transfer the creamy pumpkin sage pasta to serving plates while still hot. Garnish each plate with fresh sage leaves to enhance both the presentation and flavor of the dish.