In a small bowl, whisk together olive oil, garlic, Dijon mustard, salt, and pepper.
Place the flank steak in a resealable bag or shallow dish and pour the marinade over it. Refrigerate for 30 minutes to 2 hours.
Meanwhile, combine cherry tomatoes, mozzarella, basil, olive oil, salt, and pepper in a bowl. Toss gently and set aside.
Preheat a grill or grill pan to medium-high heat.
Remove the steak from the marinade and discard any excess.
Grill the Balsamic Caprese Grilled Flank Steak for 4–5 minutes per side for medium-rare or until your preferred doneness.
Remove from the grill and let the steak rest for 5–10 minutes.
Slice the steak thinly against the grain.
Arrange on a serving platter and spoon the Caprese mixture over the top.
Finish with a generous drizzle of balsamic glaze before serving.