Pound chicken half-breasts between sheets of wax paper until flattened. Spread mustard on top side of chicken; top with prosciutto and 2 tablespoons pistachios on each. Roll up chicken and secure with toothpicks. Mix flour with tarragon and remaining ¼ cup pistachios. Dip chicken in egg, then flour mixture, coating thoroughly. Melt 4 tablespoons butter in 13 x 9-inch baking dish or pan, place chicken in butter and bake uncovered at 375˚ F. for 20 minutes. Brush pan drippings over chicken to glaze. Add peach slices around chicken, dot with remaining 2 tablespoons butter and pour wine over. Bake 15 minutes longer or until done. Serve chicken and peaches with pan juices.