Chicken: In a small bowl, whisk together the yogurt and the 3 tablespoons vinegar until well blended. In a plastic food-storage bag, combine the chicken and yogurt mixture; seal and turn to coat. Refrigerate 20 minutes to marinate.

Meanwhile, heat a broiler, or prepare an outdoor grill with medium-hot coals, or heat a gas grill to medium-hot.

Tomato-Mozzarella Topping: In large skillet, heat the oil over medium heat. Add the onion; saute for 5 minutes or until slightly softened; the wedges will fall apart into pieces. Add the 1/4 cup balsamic vinegar and the water. Bring to a simmer. Reduce the heat to medium-low; cook until the liquid has reduced to a sauce and onion is tender, about 7 minutes.

Increase heat to medium-high. Add tomatoes, salt and pepper; cook 2 minutes. Remove skillet from heat; let cool 2 to 3 minutes. Stir in mozzarella and chopped basil.

Remove chicken from bag; discard marinade. Gently rinse chicken and pat dry.

Broil or grill chicken about 4 inches from heat 3 to 4 minutes per side or until internal temperature registers 170 degrees F on instant-read thermometer. Place chicken on large platter. Scoop 1/4 cup topping on each chicken piece. Serve immediately or let cool to room temperature. Makes 8 servings.