Cut tomato, onion, mozzarella, and basil into bite-size pieces and add to a bowl along with olive oil, salt, pepper, and balsamic vinegar. Toss everything together and taste it to make sure that the seasoning is balanced. Set aside for an hour or two.
Preheat grill to medium-high heat. Brush chicken breasts with olive oil and season with salt and pepper. Reduce grill heat to medium-low. Cook on each side for 4-5 minutes or until cooked through.
Top grilled chicken with Caprese topping and serve with your favorite side dishes.