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.
