Place the sultanas in a bowl and cover them with boiling water. Leave them to steep for 30 minutes so that they plump up and become tender, then drain.
Heat the olive oil in a large non-stick frying pan and add the well-seasoned tuna steaks. Colour them over a high heat on both sides – say, 2 minutes a side – then transfer to a plate. In the same oil, fry the celery and garlic together