Drain the tuna and place it in a medium bowl. Add the chopped onion, celery, minced garlic, and parsley. Mix thoroughly for about 2 minutes.

Incorporate mayonnaise and olive oil into the tuna mixture. Season with salt and pepper. Stir until smooth and creamy.

Heat a non-stick skillet over medium heat for about 3 minutes. Add 1 tablespoon of olive oil once hot.

Butter one side of a slice of bread and place it butter-side down in the skillet. Spoon about 2 tablespoons of the tuna mixture onto the bread.

Butter another slice of bread and place it butter-side up on top of the filling.

Grill the sandwich for about 4-5 minutes until golden brown. Flip it carefully and toast for another 3-4 minutes.

For a crispier option, use a panini press or air fryer at 370°F for 5-7 minutes.

Remove from the skillet, slice in half, and serve with salad or chips.