This curry balances the sweetness of prawns with the tang of tamarind and the warmth of mustard and fenugreek. Be careful not to overcook the prawns, as they can become rubbery. Add the tamarind water only after they are nearly done to preserve their tender texture. Leftovers taste even better the next day as the flavours meld; simply reheat gently on low heat, adding a splash of water if the gravy has reduced too much.