Cockles are a vastly underrated shellfish, full of flavour, on a par with any good clam – and a fraction of the price, too. If you can't get hold of good cockles, then buy surf or palourde clams. Before you make the soup, run the cockles under slow running water for 20 minutes or so, agitating them with your hand or a spoon.
500-600g live cockles
150ml dry cider
1 leek, halved, cut into rough 1cm dice and washed
A couple of good knobs of butter
tbsp plain flour
500ml fish stock
200ml double cream
Salt and freshly ground black pepper
2tbsp chopped green herbs (such as parsley, chervil and chives)
Drain and rinse the cockles (or the surf or palourde clams) in cold water. Place them in a saucepan with the cider, cover and cook over a medium heat for 3-4 minutes until they have just opened, shaking the pan as they are cooking, then drain in a colander over a pan to reserve the cooking liquid.
Meanwhile, heat the butter in a large thick-bottomed pan and gently cook the leeks for 2-3 minutes, stirring every so often.
Add the flour and stir well, then gradually add the cider cooking liquid and the fish stock, whisking as you are adding it to avoid lumps forming, season lightly and cook on a high heat until it has reduced by about half.
Add the cream and continue simmering until the liquid has reduced by half again. Check the seasoning, add the herbs and cockles and bring back to the boil.