Preheat the broiler or light a barbecue. Brush the tuna generously with olive oil on both sides, then broil the tuna over a medium-hot broiler or over a barbecue on both sides until browned on the outside and still pink in the middle.
Meanwhile, heat about 4tablespoons of olive oil in a small saucepan. Add the garlic and celery and fry gently until the garlic and celery have softened.
Add the olives, capers, and golden raisins, and cook over a low heat for 2 to 3 minutes.
Add the vinegar and cook over a high heat for two to three minutes to evaporate the fumes of the vinegar. Season with salt and pepper to taste and add the chopped mint.
Arrange the cooked tuna on a platter, pour over the sauce and serve immediately.