Rub the tuna with the oil and season well. Heat a large griddle pan until smoking hot, then add the tuna and cook for 2–3 minutes on each side until browned but still pink in the centre. Remove from the pan and cut into small bite-sized cubes.
Meanwhile, make the dressing by mixing together all the ingredients in a bowl.
Place the tuna on a serving plate, then sprinkle over the olive dressing. Drizzle with a little extra olive oil before serving.