In a large pot, bring salted water to a boil. Add the fettuccine and cook according to package instructions until al dente. Reserve 1 cup of pasta water, then drain the pasta and set aside.
In a large skillet, heat olive oil over medium heat. Add the chopped onion and sauté until translucent, about 5 minutes. Stir in the minced garlic and dried sage, cooking for an additional minute until fragrant.
Reduce the heat to low and add the pumpkin puree and heavy cream to the skillet. Stir well to combine, then bring to a gentle simmer. Mix in the grated Parmesan cheese, and continue stirring until the cheese is melted and the sauce is creamy. If the sauce is too thick, gradually add reserved pasta water until you reach the desired consistency.
Add the cooked pasta to the skillet, tossing it gently in the creamy pumpkin sauce until fully coated. Season with salt, pepper, and crushed red pepper flakes if using.
Plate the creamy pumpkin sage pasta while hot, and garnish with fresh sage leaves for a touch of color and herbaceous aroma.