1. Cook the pasta: Bring a large pot of salted water to a boil. Cook rigatoni according to package directions until al dente. Reserve ½ cup pasta water, drain, and set aside.
2. Brown the sausage: Heat 1 tablespoon olive oil in a large skillet over medium heat. Add chicken sausage, breaking it into small pieces. Cook for about 5 minutes or until fully cooked (165°F). Remove from the skillet and set aside.
3. Sauté the aromatics: Add the remaining tablespoon of olive oil to the same skillet. Add diced shallot and minced garlic. Cook for 2 minutes, stirring frequently, until softened and fragrant.
4. Prepare the pumpkin sauce: Stir in tomato paste and cook for 1 minute. Add pumpkin puree, heavy cream, salt, black pepper, chili flakes, nutmeg, and parmesan cheese. Stir until smooth and simmer gently for 2 minutes. Turn off the heat.
5. Make the optional topping: Melt butter in a small skillet over medium-low heat. Whisk frequently for 6–7 minutes until golden brown. Add chopped walnuts, fresh sage, and kosher salt. Cook for approximately 5 minutes until toasted and fragrant. Remove from heat.
6. Combine: Add cooked rigatoni and sausage to the pumpkin sauce along with ¼ cup reserved pasta water. Toss until coated. Add more pasta water as needed for a silky, creamy consistency. Adjust seasoning to taste.
7. Serve: Divide pasta among bowls. Top with brown butter walnuts and sage, extra parmesan cheese, and chopped fresh parsley. Serve warm.