Place fish in a fish kettle or large saucepan and cover with water. Add onion, lemon slices and wine. Bring to boil, turn down heat and simmer gently with lid on for about 25 minutes. Fish is cooked when it flakes when tested with a fork. Allow to cool and drain well.
Using a sharp knife, remove skin by making a shallow cut near head, take hold of skin and pull gently until all skin is removed. To serve segment fish with a sharp knife and lift flesh with two spoons.