In a bowl, whisk together balsamic vinegar, olive oil, garlic, Dijon mustard, salt, and pepper. Place the flank steak in a resealable plastic bag or shallow dish, and pour the marinade over it. Seal or cover, then refrigerate for at least 1 hour (up to 4 hours for deeper flavor).

Heat your grill or grill pan to medium-high. Lightly oil the grates if necessary.

Remove the steak from the marinade, letting the excess drip off. Grill for 5–7 minutes per side, depending on thickness and desired doneness—130°F (54°C) for medium-rare, 140°F (60°C) for medium.

Transfer steak to a cutting board and let it rest for 5–10 minutes. This allows the juices to redistribute. Then slice thinly across the grain for maximum tenderness.

In a small bowl, gently toss the halved cherry tomatoes, mozzarella balls, and chopped basil together with a pinch of salt and pepper.

Arrange the steak slices on a serving platter or individual plates. Spoon the Caprese mixture over the steak, and finish with a generous drizzle of balsamic glaze.