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 thoroughly browned, approximately 5–6 minutes.
02 - Incorporate the finely chopped yellow onion and cook until softened, about 3 minutes. Add minced garlic and sauté for 1 minute until fragrant.
03 - Stir in pumpkin purée, dried sage, ground nutmeg, and dried chili flakes (if using). Pour in chicken broth and water. Bring mixture to a simmer.
04 - Add rigatoni to the simmering liquid, ensuring all pasta is submerged. Continue to simmer uncovered, stirring occasionally, for 12–14 minutes, or until the pasta reaches al dente and most of the liquid has been absorbed.
05 - Reduce heat to low. Stir in heavy cream, grated Parmesan cheese, and baby spinach (if using). Cook, stirring gently, until the spinach has wilted and the sauce has achieved a creamy consistency, about 2–3 minutes.
06 - Season with salt and black pepper to your preference. Serve immediately, garnished with additional Parmesan cheese.