In a large pot of salted boiling water, cook the rigatoni according to package instructions until al dente. Drain and set aside, reserving about 1 cup of the pasta water.
In a large skillet, heat the olive oil over medium heat. Add the Italian sausage, breaking it apart with a wooden spoon. Cook until browned and cooked through, about 5-7 minutes.
Stir in the chopped onion and minced garlic to the skillet with the sausage. Sauté for about 3-4 minutes until the onion is translucent and fragrant.
Pour in the chicken broth and bring to a simmer. Then add the heavy cream, stirring to combine. Allow the sauce to cook for about 3-4 minutes, letting it thicken slightly.
Add the fresh spinach to the skillet and stir until wilted. If the sauce is too thick, gradually add some of the reserved pasta water until the desired consistency is reached.
Add the cooked rigatoni to the skillet, tossing to coat the pasta thoroughly in the creamy sauce. Stir in the grated Parmesan cheese, red pepper flakes (if using), and season with salt and pepper to taste.
Remove from heat and let it sit for a minute before serving.
Notes
Serve in deep bowls, garnished with fresh basil and extra Parmesan cheese.