01 - Preheat the oven to 375°F. Grease a 9x13-inch baking dish with cooking spray or butter.
02 - Bring a large pot of salted water to a boil. Add rigatoni and cook 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 in a single layer and cook until golden and cooked through, 5–7 minutes. Transfer to a plate.
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 the raw flour taste.
05 - Slowly whisk in whole milk and chicken broth, stirring until smooth. Add Italian seasoning and red pepper flakes. Simmer over medium heat, whisking frequently, until sauce thickens enough to coat the back of a spoon, 3–5 minutes.
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. Fold gently until pasta and chicken are evenly coated.
08 - Transfer mixture to the prepared baking dish. Spread evenly and top with remaining ½ cup mozzarella cheese.
09 - Bake for 20–25 minutes until the sauce is bubbly around the edges and the cheese topping is golden brown.
10 - Remove from oven and let rest for 5 minutes to set. Garnish with chopped fresh parsley before serving.