Trim flank steak of excess fat and fibrous membrane and place in a gallon-sized resealable plastic bag. In a medium bowl, mix the garlic, oil, balsamic vinegar, soy sauce and pepper.
Pour marinade over flank steak, remove as much air as possible from the bag and seal. Knead bag to ensure marinade is in contact with all surfaces of the meat. Place bag in a bowl or shallow dish to catch any drips and refrigerate for at least 6 hours and up to overnight.
Preheat grill to medium-high and brush grate with oil. Remove flank steak from marinade and discard bag and excess marinade.
Grill 5–6 minutes per side for medium, or until meat reaches desired doneness.
Remove from grill, cover meat loosely with aluminum foil. Allow meat to rest for 5–10 minutes, then cut into thin slices against the grain and serve.
One Last Thing
Lightly score the top and bottom of the flank steak to seal in the marinade and keep it from curling on the grill!