In a shallow dish or resealable bag, combine 2 tablespoons of olive oil, balsamic vinegar, minced garlic, salt, and pepper.
Place the flank steak in the marinade, coating thoroughly, and let sit at room temperature for 20 minutes.
Preheat the grill to medium-high heat (400°F to 450°F).
Grill the steak for 5 to 7 minutes per side, or until the internal temperature reaches 130°F for medium-rare.
Transfer the steak to a cutting board and let rest for 10 minutes to retain juices.
Arrange the tomato slices, mozzarella slices, and basil leaves on a large platter, alternating and overlapping them.
Drizzle the salad with the remaining 2 tablespoons of olive oil and the balsamic glaze, then sprinkle with flaky sea salt.
Slice the flank steak thinly against the grain at a 45-degree angle.
Arrange the steak slices on the platter next to the Caprese salad and serve immediately.