Mix all the marinade ingredients well in a bowl.
Marinate the flank steak in half of the balsamic marinade in the fridge overnight.
Shake off the excess marinade and grill the steak over medium-high heat until cooked to the desired level of doneness, about 3-5 minutes per side.
Set aside to rest for 5 minutes.
Grill the cherry tomatoes until lightly charred, about 3-5 minutes. Alternatively, blister them in a heavy-bottom skillet on the stovetop at medium-high heat.
Slice the steak thinly against the grain.
Top with the blistered tomatoes, diced mozzarella, basil, and the remaining marinade/dressing.
