Heat the broiler or the grill to high. Brush the chicken with olive oil and season with salt and pepper on both sides. Broil or grill for 6-8 minutes on each side - if using the broiler, put chicken about 4" below the heat (Chicken is done when no longer pink juices run clear or 165 degrees F in the centre with a meat thermometer).

Top the chicken with the basil leaves and slice of mozzarella, and grill or broil for a minute or so, just until cheese begins to melt. You can grill or broil the slice of tomato for a minute or so on each side as well. Then top chicken with tomato place on the plate, and drizzle with vinegar. Adjust seasoning with salt and pepper.