Place each chicken piece, boned side up, between 2 pieces of plastic wrap. Pound chicken lightly until 1/4" thick. Remove plastic wrap.
In a shallow dish, stir together flour & pepper. Lightly coat chicken with flour mixture.
In a large skillet, cook chicken in hot butter over medium heat for 4-6 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 onions. Serve with chicken & sauce.