Prepare the Marinade:
Mix all the marinade ingredients well in a bowl.
Marinate the Steak:

Marinate the flank steak in half of the balsamic marinade in the fridge overnight.
Grill the Steak:
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 Tomatoes:

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.
Assemble the Dish:
Slice the steak thinly against the grain. Top with the blistered tomatoes, diced mozzarella, basil, and the remaining marinade/dressing. Enjoy!