Wash the mussels in cold water. Scrape off any barnacles attached to the shells and pull off the brown wispy beards, if there. Discard any mussels with broken shells. If any mussels are open, tap them sharply. If they don’t close, discard them – they are dead and not edible. Rinse the mussels again, then drain in a colander. 

Check that all the clam shells are undamaged and tightly shut (or close when tapped). Rinse under cold running water to remove any grit or sand.

Put the mussels and clams in a pan large enough to hold them comfortably without crowding – it’s better if they are spread out rather than being piled on top of each other. Add the white wine, thyme and 1 of the garlic cloves. Crank up the heat, cover and steam until the shells open. Leave to cool, then remove the clams and mussels from their shells, discarding any that haven’t opened. Strain the juices and reserve.

Melt the butter in the same pan and add the carrots, leek, celery, remaining garlic, pancetta, if using, and bay leaves. Cook gently until the vegetables are softened. Add the reserved shellfish juice and the cream and bring to a simmer. Add the diced potatoes and simmer for 8–10 minutes or until they start to break down.

Add the raw fish and prawns and cook for 5–6 minutes, then add the mussels and clams to warm through for a few minutes. Finish with plenty of chopped parsley and season to taste.