Combine Worcestershire, vinegar, Dijon and pepper.
In large, shallow non-aluminum baking dish or plastic bag, pour marinade over flat iron; turn to coat. Marinate in refrigerator for 30 minutes. Remove steak; discard marinade.
Preheat grill or broiler to medium high.
Season flat iron with salt and pepper to taste. Grill or broil to desired doneness. Remove steak and let rest 5 minutes; slice thinly across the grain.