01 - Preheat the oven to 375°F. Grease a 9x13-inch baking dish.
02 - Cook the rigatoni in a large pot of salted boiling water until al dente. Drain and set aside.
03 - Season the diced chicken with salt and pepper. Heat olive oil in a large skillet over medium-high heat. Add the chicken and cook until fully cooked and lightly golden, about 5–7 minutes. Transfer to a plate.
04 - In the same skillet, melt butter over medium heat. Add minced garlic and cook until fragrant, about 1 minute. Stir in the flour and cook for 1–2 minutes.
05 - Gradually whisk in the milk and chicken broth, stirring continuously until smooth. Add Italian seasoning and red pepper flakes if using. Simmer until the sauce thickens, about 3–5 minutes.
06 - Remove from heat. Stir in mozzarella, cheddar, and Parmesan cheeses until melted and smooth.
07 - Add the cooked chicken and rigatoni to the sauce. Mix until everything is evenly coated.
08 - Transfer the mixture to the prepared baking dish. Top with remaining mozzarella cheese.
09 - Bake for 20–25 minutes, or until the casserole is bubbly and the top is golden.
10 - Garnish with chopped fresh parsley, if desired. Let rest for 5 minutes before serving.