Preheat oven to 350 degree.
Drain tuna.
Mix together tuna, celery, onion, mayo and mustard.
Place bread slices on a foil lined baking sheet, and spread tuna onto bread slices.
Top with cheddar cheese and optional sliced tomatoes.
Bake for 15 minutes or until cheese is lightly browned and bread is toasted.
Serve with baked fries and a salad.