To prepare rub, heat a small skillet over medium heat. Add peppercorns; cook 1 minute or until fragrant. Transfer toasted peppercorns to a spice grinder; grind until fine. Combine ground peppercorns, salt, mustard, garlic powder, and thyme in a small bowl. Rub peppercorn mixture evenly over steak. Let stand for 10 minutes.
Heat a nonstick grill pan over medium-high heat. Coat the flank steak with cooking spray; grill 7 minutes on each side or until desired degree of doneness. Let stand 5 minutes before slicing. Garnish with chopped parsley, if desired.