Cockle and mussel chowder with sweet potato and spring onions

Method

1. Heat the butter in a heavy-based pan, then fry the onion over a gentle heat for 10 minutes until soft, without browning. Add the garlic, thyme and spices, then fry for 1 minute. Add the leek and three quarters of the sweet potatoes. Pour in the stock. Simmer for 15 minutes, stirring occasionally.
2. Meanwhile heat a separate lidded pan over a high heat, then add the mussels. Pour in the wine, then cover for 2-3 minutes until the mussels have steamed open. Remove with a slotted spoon and set aside. Add the cockles to the pan. Cover and cook for 2-3 minutes, then add to the mussels, reserving the liquid. Leave to cool.
3. Remove the meat from half the mussels and cockles and leave the rest of the shellfish in their shells.
4. Cook the remaining sweet potato in salted boiling water for about 7 minutes, then drain and set aside.
5. Once the sweet potato in the soup is soft, blend the mixture with a stick blender or in a food processor until smooth, then pass through a sieve. Thin the soup with the shellfish juices to the desired consistency and strength of flavour.
6. To serve, add the sweet potato chunks and the shelled mussels and cockles to the soup, then reheat. Season with salt and pepper to taste. Top with the shell-on cockles and mussels, and scatter with chopped spring onions and chives.