Tuna with Sweet & Sour Onions

1½ - 2 lb tuna steaks, about ¾ " thick
salt and pepper
4 tbsp olive oil
2 large onions, thinly sliced
1 level tbsp caster sugar
3 tbsp red wine vinegar
3 fl oz dry white wine
2 tbsp chopped parsley

Divide the tuna into four portions and season with salt and pepper. Warm 3 tbsp of the olive oil in a wide frying pan and add the onions. Cook gently until soft then raise the heat to medium and fry until richly browned. Scoop out and reserve. Add the remaining oil to the pan and heat well. Fry the tuna steaks briskly on both sides to brown. Reduce the heat and cook for a further minute. Now add the sugar, vinegar and wine and return the onions to the pan. Stir around the tuna and turn the heat up. Cover and simmer for 2 more minutes. Sprinkle with parsley and serve.