1. 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.
2. Place on an ungreased baking sheet. Bake, uncovered at 350°F for 12-15 minutes or until heated through and cheese is melted. Top each with a lettuce leaf, replace bun tops.