In a mixing bowl, combine balsamic vinegar, olive oil, Dijon mustard, honey, and minced garlic. Whisk until smooth. Set aside half for later use.

Place flank steak in a shallow dish and pour marinade over, coating both sides. Cover and refrigerate for at least 2 hours.

Preheat grill to medium-high heat. Remove flank steak from marinade and grill for 3-5 minutes on each side until desired doneness. Let rest for 5 minutes.

Grill cherry or grape tomatoes on skewers for 3-5 minutes until blistered.

Slice flank steak against the grain, arrange on platter, and top with blistered tomatoes, mozzarella, and basil. Drizzle reserved marinade.