Bring 2 quarts of water to a boil in a large saucepan and cook the pasta according to package directions until it is al dente. Drain the pasta (reserve about ¼ cup of pasta water) and set it aside as you prepare the sauce.
Heat the oil in a large saute pan on medium-high heat.
Add the onion and saute for 3-4 minutes until light golden brown. Add the garlic.
Remove the sausage from the casings, add it to the pan and break up with a spatula. Cook sausage until browned and no longer pink.
Reduce the heat to medium, and add the tomato paste, dried sage, and nutmeg. Stir to combine.
Add the pumpkin puree and chicken stock. Stir the sauce to combine. Let the sauce heat through until it starts to bubble. Reduce the heat to low, add the cream and stir again.
Turn off the heat and add the 2 tablespoons of Parmesan cheese. Add salt to taste.
Fold in the pasta, and if the sauce needs to be loosened a bit, add pasta water or extra chicken stock, a few tablespoons at a time until you reach the desired consistency.
Top the pasta with chopped toasted hazelnuts, minced fresh sage and parsley. Serve with additional Parmesan cheese if desired.