Rinse and check over the clams, discarding any damaged shells or any open ones that don't close when tapped with a knife. Transfer to a bowl.
Melt the butter in a large saucepan and gently fry the onions for 6–8 minutes until soft. Add the wine and bring to the boil. Tip in the clams and cover with a lid. Cook for about 5 minutes until the clams have opened, shaking the pan several times during cooking.
Once the shells are all opened, remove from the heat and tip into a colander set over a large bowl to catch the juices. When cool enough to handle, remove the clams from the shells and discard the shells. Reserve the clams and tip the cooking juices back into the pan.
Add the stock, curry paste, turmeric and potatoes to the saucepan and bring to the boil. Reduce the heat, cover and simmer for 10–15 minutes until the potatoes are tender.
Return the clams to the pan with the watercress, nutmeg and lemon juice and heat through gently for 2 minutes. Use a stick blender to lightly blend the chowder without completely puréeing it. Season with salt and pepper to taste.