In a small bowl, whisk together the balsamic vinegar, olive oil, minced garlic, salt, and pepper. Place the flank steak in a shallow dish or resealable plastic bag and pour the marinade over it. Make sure the steak is well coated. Marinate in the refrigerator for at least 1 hour, or up to 4 hours for more flavor.

Preheat your grill to medium-high heat. Remove the steak from the marinade and let it come to room temperature for about 15 minutes. Discard the marinade.

Grill the steak for about 5-7 minutes on each side for medium-rare, or until it reaches your desired doneness. Use a meat thermometer for accuracy; it should read 130°F for medium-rare.

While the steak is grilling, combine the cherry tomatoes, mozzarella, and basil in a bowl. Drizzle with a little olive oil and season with salt and pepper to taste.

Once the steak is done, remove it from the grill and let it rest for about 5 minutes. Slice the steak against the grain into thin strips.

Serve the sliced steak topped with the Caprese mixture. Drizzle with additional balsamic vinegar if desired.