To poach the fish, lay the fillets flat in a large frying pan. Pour the entire bottle of wine over the fillets and bring to a boil. Turn the fillets over at least once. When they begin to break apart easily (about 4 min), remove fish pieces from wine with a slotted spoon and set aside.
In a separate frypan, cook the onions in olive oil over medium heat until soft. Add tomatoes, fresh parsley and cooked onions to wine. Boil vigorously for at least 10 min to boil off alcohol.
Add fish pieces and heat through. Serve with a loaf of crusty french bread and a glass of wine.