Preheat your oven to 350 degrees Fahrenheit. While you wait, prepare your 9×13-inch baking dish by spraying it with cooking spray. This ensures that your Chicken Spinach Rigatoni doesn't stick and can easily be served once it’s baked.
Cook rigatoni in a large pot of salted boiling water according to the package directions until it is al dente. Make sure to stir occasionally to prevent sticking. Once done, drain the pasta and set it aside. You want it slightly firm, as it will continue to cook in the oven.
While your pasta cooks, melt butter in a large skillet over medium heat. Add the diced onion and sauté until they are soft and translucent, about 5 minutes. This will develop a sweet flavor that forms the base of your sauce.
Next, add the chive onion cream cheese and heavy cream to the skillet. Stir frequently until the cream cheese melts and integrates with the heavy cream. You're looking for a smooth and creamy consistency.
Once the sauce is ready, mix in the salt, pepper, and crushed red pepper flakes for flavor. Then add the diced tomatoes, spinach, and cooked chicken. Remove the skillet from heat and stir well to combine all the ingredients.
Now, it's time to bring everything together! Add the cooked rigatoni to the skillet and mix until all the pasta is well coated with the creamy sauce. At this point, you can also fold in half of the mozzarella and half of the Parmesan cheese, allowing it to melt slightly into the pasta.
Transfer the mixture into the prepared baking dish, spreading it evenly. Top it off with the remaining mozzarella and Parmesan cheese, which will create a delicious cheesy crust on top as it bakes.
Place the baking dish in the preheated oven and bake for 20 minutes. You'll know it's ready when the cheese is melted and bubbly, with a slight golden color on top.
Once baked, remove from the oven and let it cool for a few minutes before serving. This allows the sauce to set a bit, making it easier to dish out.
Enjoy your homemade Chicken Spinach Rigatoni with your favorite side salad or garlic bread for a complete meal!