Gently saute onion in heated olive oil in a large, heavy pan, until soft and transparent.
Peel tomatoes and chop coarsely.
Add tomatoes, parsley and garlic to pan, cover and simmer for 15 minutes.
Meanwhile, prepare fish by filleting, removing skin and cutting into bite-size pieces.
Place fish skins and bones in a pan with 2 1/2 cups cold water.
Cover, bring to the boil and simmer for 15 minutes.
Strain off fish stock and discard bones and skin.
Add fish pieces and stock to tomato mixture, cover and bring to the boil.
Simmer for 10-15 minutes, or until fish is tender.
Season to taste with salt and pepper.
Add wine, if used, and return soup to a gentle simmer before serving.