1. Bring a large pot of salted water to a boil and cook the rigatoni until al dente. Reserve 1/2 cup of pasta water, then drain the pasta and set it aside.

2. Heat the olive oil in a large skillet over medium heat. Add the Italian sausage and cook, breaking it apart, until browned and fully cooked.

3. Add the chopped onion and cook for 3 to 4 minutes until softened. Stir in the garlic and cook for 30 seconds.

4. Mix in the tomato paste, Italian seasoning, and red pepper flakes. Stir well and let it cook for 1 minute.

5. Pour in the chicken broth and heavy cream. Stir until smooth, then let the sauce simmer for 3 to 5 minutes until slightly thickened.

6. Reduce the heat to low and stir in the Parmesan cheese until melted and creamy.

7. Add the spinach, salt, and black pepper. Stir until the spinach wilts.

8. Add the cooked rigatoni to the skillet and toss until evenly coated. Add a splash of reserved pasta water if needed to loosen the sauce.

9. Top with chopped parsley and serve hot.