1) Heat a grill pan or griddle with cooking spray over medium-high heat to preheat. Place peppercorns in a plastic bag and smash with the flat side of a meat mallet, or bottom of a heavy skillet, until finely crushed. Season both sides of flank steak with crushed peppercorns, salt and garlic flakes.
2) Place steak on hot pan and cook 5 minutes per side for medium doneness. Remove from heat and let steak rest 10 minutes. Slice meat crosswise into 1/2-cm thick slices.
3) Whilst the steak is cooking, Place diced bacon in a large skillet over medium heat until browned and crisp. Add vinegar and bring to a simmer for 5 minutes, or until liquid reduces. In a small bowl, add broth and cornflour, whisking until cornflour is completely dissolved, and add to skillet. Simmer 2 minutes bringing to a full boil, until sauce thickens. Remove from heat, spoon sauce over steak slices and top with parsley.