Bring a large pot of salted water to a boil. Add the fettuccine and cook according to package instructions until al dente. Reserve about 1/2 cup of the pasta water, then drain the pasta and set aside.
Heat olive oil in a large skillet over medium heat. Add the Italian sausage, breaking it up with a wooden spoon. Cook until browned and cooked through, about 6-8 minutes. Remove the sausage from the pan and set aside, leaving the flavorful drippings behind.
In the same skillet, add the minced garlic and sauté for about 30 seconds until fragrant. Stir in the pumpkin puree, mixing it with the garlic and sausage drippings for about 2 minutes.
Pour in the heavy cream, stirring continuously. Add the dried sage, nutmeg, salt, and pepper. Let the sauce simmer gently for 3-5 minutes until it thickens slightly.
Return the cooked sausage to the skillet and stir to combine. Add the drained fettuccine to the sauce, tossing to coat evenly. If the sauce feels too thick, add a splash of the reserved pasta water to loosen it.
Remove the skillet from heat and stir in the grated Parmesan cheese until melted and incorporated.
Plate the pasta and garnish with fresh sage leaves. Serve immediately for the best flavor and texture.