**Prepare the Marinade:**
In a mixing bowl, whisk together balsamic vinegar, olive oil, garlic, honey, Dijon mustard, salt, and pepper. Place the flank steak in a resealable bag or shallow dish and pour the marinade over it. Seal and refrigerate for at least 2 hours or up to overnight.
**Preheat the Grill:**
Preheat your grill or grill pan to medium-high heat. Oil the grates to prevent sticking.
**Grill the Steak:**
Remove the steak from the marinade, letting any excess drip off. Grill for 4-6 minutes per side for medium-rare, or adjust based on your desired doneness. Use a meat thermometer to check: 130°F (54°C) for medium-rare.
**Rest the Steak:**
Transfer the steak to a cutting board and let it rest for 5-10 minutes to retain its juices.
**Prepare the Caprese Topping:**
In a bowl, toss the cherry tomatoes with a pinch of salt and pepper.
**Slice and Assemble:**
Slice the steak thinly against the grain. Arrange the slices on a serving platter, top with fresh mozzarella, cherry tomatoes, and basil leaves.
**Drizzle with Balsamic Glaze:**
Finish the dish with a drizzle of balsamic glaze. Serve immediately.