Scrub the shellfish and leave to soak in a bowl of water for an hour.
To make the sauce pour the oil into a large wide lidded pan and set over a medium heat. When the oil is hot add the onions and stir and fry for about 5 minutes until translucent, reducing the heat if needed.
Add the ginger and garlic and stir for a minute. Add the cayenne pepper, paprika, turmeric, cumin and coriander. Stir for 10 seconds then add 500 ml water together with the tamarind, salt and coconut milk.
Stir and bring to a simmer then cover, reduce the heat to low and simmer gently for 20 minutes.
Add the clams or other shellfish and bring to a simmer. Cover and simmer vigorously for about 5 minutes until all the clams have opened. Serve immediately with rice.