1. Place each chicken breast half between 2 sheets of heavy-duty plastic wrap; pound to 1/4-inch thickness using a meat mallet or small, heavy skillet. Sprinkle both sides of chicken with 1/4 teaspoon pepper and salt. Dredge chicken in flour, shaking off excess.
2. Heat 1 teaspoon oil in a large nonstick skillet over medium heat. Add chicken, and cook 4 to 5 minutes on each side or until done. Remove chicken from pan, and keep warm.
3. Heat remaining 1 teaspoon oil in pan. Add shallots; sauté 1 minute. Add wine and broth; bring to a boil. Boil 3 to 4 minutes or until mixture is reduced by half. Add mustard, tarragon, and remaining 1/4 teaspoon pepper; cook 1 minute. Remove from heat; stir in sour cream.