01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with butter or cooking spray.
02 - Bring a large pot of salted water to boil. Cook rigatoni until al dente according to package directions. Drain well and set aside.
03 - Season diced chicken with 1 tsp salt and ½ tsp black pepper. Heat olive oil in a large skillet over medium-high heat. Add chicken and cook 5-7 minutes until fully cooked and lightly golden. Transfer to a plate.
04 - In the same skillet, melt butter over medium heat. Add minced garlic and cook 1 minute until fragrant. Stir in flour and cook 1-2 minutes, stirring constantly to eliminate raw flour taste.
05 - Gradually whisk in milk and chicken broth, stirring continuously until smooth. Add Italian seasoning and red pepper flakes. Simmer 3-5 minutes until sauce thickens and coats the back of a spoon.
06 - Remove skillet from heat. Stir in mozzarella, cheddar, and Parmesan cheeses until completely melted and smooth.
07 - Add cooked chicken and rigatoni to the cheese sauce. Mix thoroughly until pasta and chicken are evenly coated.
08 - Transfer mixture to prepared baking dish. Spread evenly and top with remaining ½ cup mozzarella cheese.
09 - Bake 20-25 minutes until casserole is bubbly around edges and cheese topping is golden brown.
10 - Remove from oven and let rest 5 minutes before serving. Garnish with chopped fresh parsley if desired.