Place chicken breast into a pot with boiling water. Simmer until the chicken reaches an internal temperature of 165 degrees or until chicken is no longer pink inside. Remove chicken from water and shred or cut into small pieces. Set aside.
Chop onions and mince garlic and saute in a pan sprayed with cooking spray until they are cooked. Set aside
Cook penne pasta according to package directions.
Drain pasta and place it into a large bowl along with chicken, Alfredo sauce, ¾ cup of mozzarella cheese, onions and garlic. Mix until well combined.
Pour into a 9x13 casserole dish. Top with remaining mozzarella cheese and shredded cheddar.