Preheat the oven to 190 degrees C. Place bread slices in the oven to toast while it preheats, and while you make the tuna salad.
In a small bowl, mix together the tuna, mayonnaise, salt, balsamic vinegar, mustard and dill pickle until well blended. Remove bread from the oven, and pile the tuna mixture onto one slice. Sprinkle cheese over the other slice of bread.
Bake for 7 minutes in the preheated oven, or until cheese is melted and tuna is heated through. Place the cheese side of the sandwich on top of the tuna side. Cut in half and serve immediately.