In a gallon zip top bag, combine all of the ingredients for the rosemary balsamic marinade. Zip the top, then toss gently until all of the salt and sugar granules are dissolved.
Place the flank steak in the marinade, press out any excess air from the bag, and seal tightly. Using your hands, massage the marinade around the meat. Refrigerate for at least 2 hours, or up to 24 hours.
Preheat your grill for direct heat grilling at 450 degrees F. Grill the steak for 4-5 minutes per side, or until an internal thermometer reads 130-135 degrees F for medium rare. Remove the steak from the grill to a cutting board and allow to rest for 10-15 minutes.
Slice the steak into thin strips against the grain (perpendicular to the long strands of muscle) and enjoy!