Combine oil, lemon juice and pepper in shallow dish. Swirl chicken in mixture to coat. Cover with plastic wrap and marinade in frig 30 minutes. Meanwhile combine vinegar and wine in heavy small sauce pan and boil over medium high heat until liquid is reduced to about 2 Tablespoons. Remove from heat and add tarragon and pepper. Whisk in butter on tablespoon at a time blending thoroughly after each addition.
Place over low heat and continue whisking until sauce has thickened slightly. Whisk in mustard. Set aside and keep warm. Prepare charcoal grill. Drain chicken well. Grill three to four minutes on each side. Arrange on heated platter. Garnish with lemon slices and parsley. Serve warm sauce separately.