Begin by bringing a large pot of salted water to a boil. Add the rigatoni and cook according to package instructions until al dente. Drain and set aside.
In a large skillet, melt the butter over medium heat. Add the diced onion and sauté until softened and translucent, about 3-4 minutes.
Add the chive onion cream cheese to the skillet with the onions. Stir until melted and combined, then slowly pour in the heavy cream. Mix until the sauce is smooth.
Sprinkle in the salt, pepper, and crushed red pepper flakes. Stir well to incorporate all the flavors.
Fold in the drained diced tomatoes and the thawed spinach. Ensure everything is well mixed and heated through.
Add the diced cooked chicken to the skillet, stirring to combine. Finally, gently fold in the cooked rigatoni until everything is evenly coated in the creamy sauce.
Sprinkle in the shredded mozzarella cheese, mixing until melted and gooey. Transfer the mixture to a baking dish if you prefer a baked version.
Top with the freshly shredded Parmesan cheese and additional mozzarella if desired. Bake in a preheated oven at 350°F (175°C) for about 15-20 minutes, or until the cheese is bubbly and golden.