Heat a film of cooking oil in a frying pan. Flatten each fish fillet with a rolling pin. Sprinkle on some salt and pepper and fold in half. Fry gently and briefly on both sides.
Arrange the fish on a long dish and scatter the onion rings over the top. Mix the dressing ingredients together and pour over the fish whilst it is still hot. Leave for at least 3 hours to cool. Serve at room te