Preheat the oven to 375 degrees F. Bring a large pot of salted water to a boil and cook the penne until just al dente. Drain and set aside.
In a large skillet over medium heat, cook the Italian sausage, breaking it up with a spoon until browned. Add the diced onion and cook for 3 minutes until softened.
Stir in the minced garlic and cook for 1 minute. Add the pumpkin puree, heavy cream, chicken broth, sage, nutmeg, salt, and pepper. Simmer for 5 minutes until the sauce thickens slightly.
Remove the skillet from heat and stir in the cooked pasta along with 1 cup of mozzarella and all of the Parmesan until evenly coated.
Transfer the mixture to a 9x13-inch baking dish. Sprinkle the remaining 1 cup of mozzarella evenly over the top.
Bake for 20 minutes until the cheese is melted and lightly golden. Let rest for 5 minutes, then sprinkle with chopped parsley before serving.