Place each chicken breast half between two pieces of plastic wrap. Using the flat side of a meat mallet, lightly pound chicken to 1/4- to 1/8-inch thickness. Discard plastic wrap. In a shallow dish combine flour and pepper. Coat chicken pieces with flour mixture.
In a 12-inch skillet melt butter over medium heat. Add chicken to skillet. Cook about 6 minutes or until no pink remains in the chicken, turning once. Transfer chicken to a platter, reserving drippings in skillet. Cover to keep warm.
For sauce, add green onion to the drippings in the skillet. Cook and stir over medium heat for 1 to 2 minutes or until tender. Stir in whipping cream, chicken stock, and mustard. Cook and stir for 1 to 2 minutes or until smooth and slightly thickened. Spoon sauce over chicken. If desired, snip fresh thyme to sprinkle over chicken and garnish with thyme sprigs.