Preheat the oven to 400 degrees F (200 degrees C). Coat a medium-sized casserole dish with cooking spray.
Bring a large pot of lightly salted water to a boil. Cook rigatoni in the boiling water, stirring occasionally until tender yet firm to the bite, about 8 minutes. Drain and set aside.
Mix mayonnaise, garlic, onion, salt, and pepper together in a medium-sized bowl. Add 1 1/4 cup Parmesan cheese; mix well. Mix in cooked pasta and chicken, making sure everything is evenly coated.
Transfer mixture to the prepared casserole dish. Sprinkle with remaining 1/4 cup Parmesan cheese and panko bread crumbs. Cover with aluminum foil.
Bake in the preheated oven for 30 minutes. Remove from the oven, uncover, and replace in the oven. Bake until chicken is no longer pink in the center and juices run clear, about 10 minutes more. Let stand for 1 to 2 minutes before dishing out.
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