In a small bowl, whisk together balsamic vinegar, olive oil, minced garlic, honey, mustard, salt, and pepper.
Place flank steak in a sealable bag and pour in half of the marinade; refrigerate for 60 minutes.
Preheat grill to high heat (450°F / 232°C).
Remove steak from marinade and pat dry with paper towels; discard used marinade.
Grill steak for 5 to 7 minutes per side until internal temperature reaches 130°F (54°C) for medium-rare.
Transfer steak to a cutting board and allow to rest for 10 minutes.
Slice steak against the grain into thin strips.
Arrange steak on a serving platter and top with tomato slices and mozzarella.
Garnish with fresh basil and drizzle with the balsamic glaze before serving.