1. Stir together first 7 ingredients in a medium bowl; gently stir in chicken.
2. Carefully scoop out and discard tomato pulp, leaving a 1/8-inch shell. Place tomato shells upside down on several layers of paper towels; let drain 5 minutes.
3. Spoon about 1 Tbsp. chicken salad into each tomato shell. Garnish, if desired.