Heat 6 tbsps olive oil in a saute pan and add the onions and celery. Cook, stirring occasionally, for about ten minutes until the onions are golden and soft.
Add the raisins, vinegar, Marsala, olives and sugar. Season and stir to combine. Cook for a couple of minutes and check seasoning- you want a sweet and sour taste so add more vinegar or sugar as needed.
Brush the tuna steaks on both sides with the rest of the oil and season with salt and pepper. Heat a ridged griddle pan or frying pan until really hot.
Quickly sear the tuna on both sides so that it gets a good colour. Transfer the tuna to the saute pan and cook for a couple of minutes in the sauce, or longer if preferred.
Add the mint leaves then serve with a slug of extra virgin olive oil on top.