1 kg tuna, cut into six 2.5cm-thick steaks (about 160gm each)½ cup olive oil2 stalks celery, strings removed, thinly sliced 2 white onions, thinly sliced 2 fresh bay leaves1 tbsp granulated sugar½ cup red wine vinegar125 ml sweet Marsala, Madeira or dessert wine55 gm (½ cup) muscatels, cut into clusters, soaked in 1 cup warm water for 20 minutes, drained12 Sicilian green olives, pittedTo serve:green salad
Place tuna in a bowl, cover with cold water and stand for 10 minutes, then drain and pat dry with absorbent paper.
Heat ¼ cup olive oil in a large frying pan, add celery and onions and stir over medium heat for 8 minutes or until soft. Add bay leaves and sugar and cook for another 2 minutes, then transfer to a bowl.
Season tuna to taste with sea salt and freshly ground black pepper. Heat remaining oil in a large saucepan, add tuna steaks and cook, in batches, over medium heat for 4 minutes on each side or until just cooked through. Return tuna to pan, add onion mixture and remaining ingredients, then simmer over low heat for 5 minutes or until the sauce thickens. Season to taste, then serve immediately or at room temperature, with a green salad passed separately.