1. Heat a large skillet over medium-high heat. Sprinkle chicken with pepper and salt. Coat pan with cooking spray. Add chicken to pan; cook 4 minutes, browning on all sides. Remove chicken from pan; keep warm.
2. Return pan to medium-high heat. Add onion, carrot, celery, and thyme to pan. Cook 6 minutes or until vegetables are tender and lightly browned, stirring frequently. Combine stock, milk, and flour in a bowl, stirring with a whisk until well blended. Gradually add stock mixture to pan, stirring constantly; bring to a boil. Cook 2 minutes or until thickened. Stir in chicken and pierogies, and cook 4 minutes or until chicken is done and pierogies are warm. Sprinkle with chopped parsley.