Heat the oil in a large saucepan, add the onions and fry gently for 5 minutes until soft and lightly coloured. Add the tomatoes, with their juice, and the bouquet garni. Bring to the boil, then lower the heat and simmer for 5 minutes, stirring and breaking up the tomatoes with a wooden spoon.

Add all the remaining ingredients except the parsley and simmer uncovered, for 10 - 15 minutes, or until the potato is cooked.

Add the fish to the pan and simmer gently, uncovered, for about 5 minutes, or until the fish is tender but not breaking up. Remove the bouquet garni, stir in the parsley, then taste and adjust seasoning. Transfer to a warmed serving bowl and serve at once.