01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish with cooking spray or butter.
02 - Bring a large pot of salted water to boil. Cook rigatoni according to package directions until al dente. Drain well and set aside.
03 - Season diced chicken with salt and pepper. Heat olive oil in a large skillet over medium-high heat. Add chicken and cook until browned and cooked through, 5–7 minutes. Remove from skillet and set aside.
04 - In the same skillet, melt butter over medium heat. Add minced garlic and sauté until fragrant, about 1 minute. Sprinkle in flour and whisk constantly for 1–2 minutes to cook out raw flour taste.
05 - Gradually whisk in milk and chicken broth until smooth. Add Italian seasoning and red pepper flakes. Simmer sauce over medium-low heat, whisking frequently, until thickened, 3–5 minutes.
06 - Remove skillet from heat. Stir in mozzarella, cheddar, and Parmesan cheeses until completely melted and sauce is smooth and creamy.
07 - Add cooked chicken and rigatoni to the cheese sauce. Mix thoroughly until all components are evenly coated.
08 - Transfer mixture to prepared baking dish. Spread evenly and top with remaining ½ cup mozzarella cheese.
09 - Bake for 20–25 minutes until cheese is melted and bubbly, and top is golden brown.
10 - Remove from oven and let casserole rest for 5 minutes before serving. Garnish with fresh parsley if desired.