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 fully cooked, approximately 5 to 6 minutes.
02 - Add the finely chopped onion and cook until softened, about 3 minutes. Stir in the minced garlic and continue cooking 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, then bring the mixture to a gentle simmer.
04 - Add rigatoni to the simmering liquid, ensuring all pieces are submerged. Allow to simmer uncovered, stirring occasionally, for 12 to 14 minutes, until pasta is al dente and most liquid is absorbed.
05 - Reduce the heat to low. Stir in heavy cream, grated Parmesan cheese, and baby spinach if using. Cook, stirring, for 2 to 3 minutes until the spinach wilts and sauce is creamy.
06 - Season generously with salt and freshly ground black pepper to taste. Serve hot, garnishing with additional Parmesan cheese as desired.