Brush Steak With Olive Oil, Then Press Into Ground Peppercorns And Season With Salt; Set Aside. Melt Butter In A Large Skillet Over Medium High Heat. Add Flat Iron, And Cook To Desired Degree Of Doneness, About 4 Minutes Per Side For Medium-Rare. Once The Steak Has Finished Cooking, Remove To A Plate And Keep Warm. Stir The Minced Garlic Into The Skillet, And Cook Over Medium Heat Until The Garlic Begins To Brown Around The Edges, About 1 Minute. Pour In The Balsamic Vinegar, Increase Heat To Medium-High, And Simmer Until The Vinegar Has Reduced By Half. Thinly Slice The Flat Iron Steak Across The Grain, And Arrange On A Serving Platter. Pour The Reduced Balsamic Overtop, And Sprinkle With Chopped Parsley To Serve.