In a bowl, whisk together the balsamic vinegar, olive oil, Dijon mustard, honey, garlic, salt, and pepper until smooth.
Reserve half of the dressing for serving and pour the remaining half over the flank steak.
Cover and refrigerate overnight, allowing the steak to fully absorb the flavors.
Remove the steak from the marinade and gently shake off any excess.
Grill over medium-high heat for 3–5 minutes per side, or until your preferred level of doneness. This Balsamic Grilled Flank Steak is best served medium-rare to medium.
Transfer the steak to a cutting board and let it rest for 5 minutes.
Grill the cherry tomatoes for 3–5 minutes until lightly blistered. Alternatively, blister them in a hot skillet.
Slice the steak thinly against the grain.
Arrange on a serving platter and top with the grilled tomatoes, diced mozzarella, fresh basil, and the reserved balsamic dressing.
Serve immediately while warm.