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Drain the sauce from the mussels into a large saucepan and add the stock. Tip in the potatoes, cover and bring to the boil. Once boiling, take off the lid and simmer for about 12 mins or until the potatoes are very tender.
Meanwhile, cut the fish into large chunks and roughly chop the parsley. Stir the fish and mussels into the soup, then bring back to a simmer for about 3 mins or until the fish has changed colour and flakes easily. Stir in most of the parsley, then serve scattered with the rest of the parsley and eat with crusty bread.