Pat the fish dry on kitchen paper and cut into 4 pieces. Season with salt and pepper. Put the wine and stock in a saucepan and bring almost to the boil. Set aside.
Melt the butter in a large, heavy-based saucepan and sauté the onions for 5 minutes, until softened. Add the rice and sun-dried tomatoes and stir well to coat the grains with the butter. Add the hot stock mixture, a large ladleful at a time, stirring until each addition is absorbed into the rice. Continue adding stock in this way, cooking until the rice is creamy but the grains are still firm. This should take about 20 minutes.
Meanwhile, heat 2 tablespoons of the oil in a large frying pan and cook the fish for 3 minutes on each side, until cooked through. Remove with a fish slice and keep warm.
Add the oregano and tomatoes to the pan and cook for 1 minute. Season lightly with salt and pepper.
Stir the Parmesan into the risotto and pile on to plates. Top with the fish and tomatoes. Add the olives, the remaining oil and the vinegar to the frying pan, stirring for a few seconds, then pour over the fish to serve.