01 - Heat olive oil in a large, deep skillet or Dutch oven over medium heat. Add Italian sausage and cook, breaking it apart with a spoon, until well-browned and cooked through, approximately 5–6 minutes.
02 - Add the finely chopped onion to the skillet and cook until softened, about 3 minutes. Stir in the minced garlic and cook for 1 minute until fragrant.
03 - Incorporate the pumpkin purée, dried sage, ground nutmeg, and dried chili flakes (if using). Pour in the chicken broth and water. Bring the mixture to a gentle simmer.
04 - Add the rigatoni to the simmering liquid, ensuring all pasta is submerged. Continue to simmer uncovered, stirring occasionally, for 12–14 minutes, or until the pasta achieves an al dente texture and most of the liquid has been absorbed.
05 - Reduce the heat to low. Stir in the heavy cream, grated Parmesan cheese, and baby spinach (if using). Continue to cook, stirring, until the spinach has wilted and the sauce has become creamy, approximately 2–3 minutes.
06 - Season the dish generously with salt and freshly ground black pepper to your preference. Serve hot, with additional grated Parmesan cheese as a garnish.