Drain water or oil from tuna, then fork tuna into a medium bowl.
Chop onion, celery, and parsley.
Mince garlic and add ingredients to tuna.
Add mayonnaise, olive oil and mozzarella cheese.
Combine all ingredients until well incorporated. Salt and pepper to taste.
Place a non-stick skillet on the stove and add 1 tablespoon of olive oil.
When skillet is hot, take a slice of bread and generously spread butter on one side.
Place bread buttered side down in hot skillet.
Using a spoon, scoop about 2 tablespoons of tuna salad on bread while it is browning. Butter another slice of bread and place non-buttered side on top of tuna.
Utilizing a spatula, check to see if bread is toasted brown; if so, turn tuna melt over and grill until golden brown.
Set tuna melt on paper towels and allow to cool enough to serve.