In a mixing bowl, whisk together balsamic vinegar, olive oil, honey, minced garlic, salt, black pepper, and rosemary until well combined.

Place the flank steak in a large resealable plastic bag or shallow dish and pour the marinade over it. Seal the bag or cover the dish and refrigerate for at least 1 hour, or up to 8 hours for more flavor.

Preheat the grill to medium-high heat. Remove the steak from the marinade and discard the marinade.

Place the flank steak on the grill and cook for about 5-7 minutes on each side, or until the internal temperature reaches 130°F (54°C) for medium-rare.

Remove the steak from the grill and let it rest for 5 minutes before slicing against the grain.