In a bowl, stir together mayonnaise, relish, and mustard.
Add tuna, cheese, celery, and onions; stir until well combined.
Spread tuna mixtureevenly over bottom halves of rolls; top with remaining roll halves.
Serve or, to heat, wrap each sandwich in foil; place sandwiches on a baking sheet and heat in a 350° oven until cheese is melted 15 to 20 minutes.
Unwrap and serve immediately.