Cook pasta as per package instructions. Save 1 cup of pasta cooking water.
Heat oil in a skillet or pan. Saute diced onion and garlic for about 1 minute until softened.
Add ground sausage. If using link sausage, remove the casing and after adding it to the skillet break into small pieces with a wooden spatula.
Cook sausage for 5-6 minutes until light brown in colour. Add crushed fennel, chili flakes, Italian seasoning, salt and pepper. Saute for another minute.
Deglaze the pan with dry white wine. Let the white wine cook down to almost half. Then stir in the flour and cook for a minute.
Lower the heat, add heavy cream, chicken stock. Give everything a good mix. Add chopped spinach to the sauce. Simmer the sauce for 5 minutes.
Add grated parmesan and mix. Stir in the cooked rigatoni and mix to coat pasta.