Flake tuna and combine with beans, corn, cheese, dressig and onions in a medium bowl. Cover and chill 1 hour. Just before serving, core each tomato and carefully scoop out center to within 1/4 inch of edge forming a bowl; discard flesh and seeds. Drain tomatoes upside down on a paper towel. Cut each tomato into 5 or 6 wedges, leaving the bottom intact. Gently open each tomato to support the salad. Arrange tuna mixture on top of the tomatoes.