Place each chicken piece, boned side up, between two pieces of plastic wrap. Pound chicken lightly until 1/4" thick. Remove plastic wrap.
In shallow dish, stir together flour and pepper. Lightly coat chicken with flour mixture.
In large skillet, cook chicken in hot butter over medium heat for 4 to 6 minutes or until chicken is tender and no longer pink, turning once. Remove chicken from skillet, cover and keep warm.
For sauce: In a small bowl, stir together chicken broth, lemon juice and cornstarch. Add to skillet. Cook and stir until thickened and bubbly. Cook and stir for 2 minutes more. Stir in green onion. Serve chicken with sauce.