Marinate the Steak:
In a mixing bowl, whisk together balsamic vinegar, olive oil, garlic, oregano, salt, and pepper. Place the flank steak in a resealable bag or shallow dish and pour the marinade over it. Let it marinate in the refrigerator for at least 2 hours, or overnight for best results.
Prepare the Grill:
Preheat the grill or grill pan to medium-high heat. Lightly oil the grates to prevent sticking.
Grill the Steak:
Remove the steak from the marinade and discard the excess liquid. Grill the steak for 4–6 minutes per side for medium-rare, or until it reaches your desired doneness (use a meat thermometer to check: 130°F for medium-rare). Let the steak rest for 5–10 minutes before slicing.
Make the Balsamic Glaze (if needed):
While the steak is grilling, simmer balsamic vinegar in a small saucepan over medium heat. Stir occasionally until it reduces by half and thickens into a glaze. Add honey for sweetness if desired.
Assemble the Caprese Salad:
In a bowl, combine tomatoes, mozzarella, basil, olive oil, salt, and pepper. Toss gently to coat.
Slice the Steak:
Slice the rested steak against the grain into thin strips.
Plate and Serve:
Arrange the steak slices on a platter and top with the Caprese salad. Drizzle with balsamic glaze and garnish with extra basil if desired.