1. Place each chicken breast half between 2 sheets of heavy-duty plastic wrap; pound to 1/2-inch thickness using a meat mallet or small heavy skillet. Sprinkle chicken with 1 tablespoon sage, salt, and 1/8 teaspoon pepper.
2. Heat 2 teaspoons oil in a large nonstick skillet over medium heat. Add chicken to pan; cook 3 minutes on each side or until done. Remove chicken from pan; keep warm. Heat remaining 2 teaspoons oil in pan. Add onion to pan; cook 3 minutes or until tender. Stir in wine and broth; bring to a boil. Add remaining 1 tablespoon sage and dried plums to pan; cook 4 minutes or until mixture thickens. Stir in remaining 1/8 teaspoon pepper and vinegar.