In A Small Bowl, Combine The Tuna, Celery, Onion And Mayonnaise. Spread On Bottom Of Buns; Set Bun Tops Aside. Top Tuna Mixture With A Slice Of Cheese And Tomato. Place On An Ungreased Baking Sheet. Bake, Uncovered, At 350 Degrees F For 12-15 Minutes Or Until Heated Through And Cheese Is Melted. Top Each With A Lettuce Leaf; Replace Bun Tops.