Preheat oven to 375º. Butter 8 (8 oz) ramekins.
Cook sausage in a large, deep skillet over medium heat until fat is rendered about 8 minutes. Remove from skillet with a slotted spoon, drain on paper towels and set aside. Discard any fat from the skillet in excess of 2 tablespoons.
Add onion, garlic, crushed red pepper flakes and oil to the skillet and cook, stirring occasionally until soft; about 3 minutes. Stir in pumpkin puree, chicken stock and sage. Mix together and add salt. Bring to a boil then lower the heat and simmer 5 minutes. Stir in cream and sausage. Simmer until the sauce comes together and is thickened slightly.
Add cooked pasta and parsley to the skillet and gently toss all the ingredients together to coat. Divide the rigatoni mixture between the 8 prepared ramekins. Sprinkle the tops of each ramekin with Parmesan cheese and bake for 35 minutes until the topping is golden brown.