1. Marinate the flank steak by whisking together balsamic vinegar, olive oil, Dijon mustard, honey, minced garlic, salt, and pepper in a medium-sized bowl.
2. Place the flank steak in a large zip-top bag or shallow dish and pour the marinade over the steak, ensuring it is evenly coated.
3. Seal the bag or cover the dish and refrigerate for at least 4 hours or preferably overnight for best results.
4. Remove the steak from the refrigerator 30 minutes before grilling to bring it to room temperature for even cooking.
5. Preheat your grill to medium-high heat (around 400°F).
6. Pat the steak dry with a paper towel to ensure a proper sear and remove excess marinade.
7. Grill the flank steak for 3-5 minutes per side, depending on your desired doneness. Use a meat thermometer to check internal temperatures (medium-rare: 130-135°F, medium: 135-145°F, well-done: 155°F+).
8. Let the steak rest for at least 10 minutes after grilling to lock in the juices.
9. Prepare the cherry or grape tomatoes by grilling, blistering in a skillet, or roasting in the oven until lightly charred.
10. Arrange the cooked and rested flank steak slices on a platter or individual serving plates.
11. Scatter the grilled or blistered tomatoes over the steak.
12. Add fresh mozzarella, either diced or as small mozzarella balls (bocconcini).
13. Sprinkle fresh basil (torn or thinly sliced) generously on top of the dish.
14. Drizzle the balsamic dressing to finish the dish, ensuring you reserve some separately before adding it to the steak to avoid cross-contamination.
15. Serve and enjoy immediately, optionally pairing with sides like roasted vegetables, arugula, or crusty bread.