1. Place potatoes in a large saucepan; cover with water. Bring to a boil. Cook 12 minutes or until tender; drain. Return potatoes to pan; mash to desired consistency. Stir in stock, green onions, mustard, and 1/2 teaspoon pepper.
2. Sprinkle chicken with remaining 1/2 teaspoon pepper and salt. Place flour in a shallow dish. Combine milk and egg in a shallow dish. Combine panko, parsley, and garlic powder in a shallow dish. Dredge chicken in flour. Dip in milk mixture; dredge in panko mixture, shaking off excess. Lightly coat breaded chicken with cooking spray.
3. Heat a large nonstick skillet over medium-high heat. Add 2 teaspoons oil to pan; swirl to coat. Add 2 chicken breast halves to pan; cook 3 minutes on each side or until chicken is done. Remove chicken from pan; keep warm. Repeat procedure with remaining 2 teaspoons oil and remaining 2 chicken breast halves.