Cut tomato, onion, mozzarella, and basil into bite-size pieces. Add to a mixing bowl. Add a generous amount of salt and pepper.
Mix in oil and vinegar.
Give everything a toss. Taste it to make sure that the seasoning is right. Set aside for an hour or two. Taste it again before you are going to serve it.
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.