In a bowl, stir together mayonnaise, relish, and mustard.
Add tuna, cheese, celery, and onions; stir until well combined.
Spread tuna mixture evenly over bottom halves of rolls; top with remaining roll halves.
To serve hot: Wrap each sandwich in foil.
Place on a baking sheet in a 350° oven for 15 to 20 minutes or until cheese is melted.
Unwrap and serve immediately.