Bring a large pot of salted water to a boil and cook the rigatoni according to package directions until al dente. Drain and set aside.
Heat the olive oil in a large skillet over medium heat. Add the sausage and cook, breaking it up with a spoon, until browned and cooked through.
Add the diced onion to the skillet and cook for 3 to 4 minutes until softened. Stir in the minced garlic and cook for 1 minute more.
Stir in the pumpkin puree, heavy cream, dried sage, and nutmeg. Season with salt and black pepper. Simmer the sauce for 5 minutes, stirring occasionally.
Add the drained rigatoni to the skillet along with the grated Parmesan cheese. Toss everything together until the pasta is evenly coated.
Remove from heat and sprinkle with chopped fresh parsley before serving.