Bring a large pot of salted water to a boil and cook the rigatoni until al dente. Drain and set aside.
Heat a large skillet over medium heat. Add the sausage and break it into small pieces with a spoon. Cook until browned and cooked through.
Add the diced onion and minced garlic to the skillet. Stir and cook for 3 minutes until the onion softens.
Stir in the pumpkin puree, heavy cream, chicken broth, red pepper flakes, dried sage, salt, and black pepper. Bring the mixture to a gentle simmer and cook for 5 minutes.
Add the drained rigatoni and grated Parmesan cheese to the skillet. Toss everything until the pasta is evenly coated and heated through.
Remove the skillet from the heat and sprinkle with the chopped parsley before serving.