Place each chicken half between heavy plastic, pound to 1/4-inch thickness, sprinkle both sides of chicken with 1/4 tsp. each of pepper and salt. Dredge chicken in flour, shake off any excess.
Heat 1 tsp. oil in lg skillet over med heat. Add chicken, cook for 4-5 min each side, or until done. Remove chicken from skillet, keep warm.
Heat remaining oil in skillet. Add shallots, saute 1 min. Add broth, bring to boil. Boil for 4 min or until mix is reduced by half. Add mustard, tarragon and remaining 1/4 tsp. pepper. Cook 1 min, remove from heat, and stir in sour cream. Serve sauce over warmed chicken.