Preheat oven to 425. Place a rack in a roasting pan or large ovenproof skillet. Lightly oil. Mash garlic to a paste with 1/4 teaspoon salt. Combine with parsley, rosemary, lemon zest and pepper. Press breasts with hand to flatten. Place mixture between skin and flesh of chicken. Place breasts on prepared rack and toast for 35 minutes.
Transfer chicken to carving board, cover and keep warm Pour drippings from pan Into small bowl and chill in freezer. Add wine to the roasting pan and bring to boil. Boil until reduced by half. Pour in chicken stock and return to boil and reduce. Skim fat from chilled pan drippings and add to sauce. Thicken with cornstarch. Stir in mustard. Serve sauce over chicken.