Marinate the steak:
In a mixing bowl, whisk together balsamic vinegar, olive oil, minced garlic, oregano, salt, and pepper.
Place the flank steak in a large zip-top bag or shallow dish and pour the marinade over it. Seal or cover and refrigerate for at least 2 hours, or overnight for maximum flavor.
Preheat the grill:
Preheat your grill or grill pan to medium-high heat.
Remove the steak from the marinade and let it sit at room temperature for 20 minutes. Discard the marinade.
Grill the steak:
Place the steak on the hot grill and cook for 4-5 minutes per side for medium-rare, or adjust cooking time based on your preferred doneness.
Remove the steak from the grill and let it rest for 10 minutes before slicing.
Assemble the Caprese topping:
While the steak rests, prepare the Caprese topping by slicing the mozzarella, tomatoes, and gathering fresh basil leaves.
Slice and serve:
Slice the flank steak against the grain into thin strips. Arrange the slices on a serving platter.
Layer the mozzarella, tomato slices, and basil leaves on top of the steak.
Drizzle generously with balsamic glaze.