Pound chicken breasts with a meat mallet to 1/2-in. thickness. Sprinkle chicken with 1/2 teaspoon salt and pepper., In a large skillet, heat butter and oil over medium heat. Add chicken; cook 4-5 minutes on each side or until no longer pink. Remove chicken from pan; keep warm., Add shallot to same pan; cook and stir over medium heat until tender. Add cream, stirring to loosen browned bits from pan. Increase heat to medium-high; cook until slightly thickened. Stir in 2 teaspoons tarragon, lemon juice and remaining salt. Serve with chicken. Sprinkle with remaining tarragon.