Cut 1/2-inch off top of each tomato. Scoop out pulp from tomatoes. Turn tomatoes upside down on paper towels to drain.
In a medium bowl, combine chicken, bell pepper, corn, and onion.
In a small bowl, whisk together olive oil and next 6 ingredients. Pour over chicken, tossing gently to coat.
Line tomatoes with leaf lettuce or spinach leaves. Spoon chicken salad evenly into tomatoes. Refrigerate or serve immediately.