Wash the clams thoroughly. Separate the shells and retain only the shell containing the flesh. Discard the empty shell.
Dry roast the coriander seeds, cumin seeds, ½ tsp mustard seeds, carom seeds, fenugreek seeds, peppercorns and dry red chillies until aromatic. Allow to cool.
Transfer the roasted spices to a mixer jar. Add coconut, garlic, 1 chopped onion, turmeric, tamarind and a little water. Grind to a smooth paste.
Heat oil in a pan. Add the remaining ½ tsp mustard seeds. When they splutter, add curry leaves and the remaining onion. Sauté until the onion softens.
Add the ground masala and cook for a minute. Add water as required and salt to taste. Bring the curry to a boil.
Add the clams and mix gently. Cover and cook for 8–10 minutes or until the clams are cooked.
Add the chopped kappa rotti pieces and gently mix.
Switch off the heat and keep the pan covered for 15 minutes. During this time, the kappa rotti absorbs all the flavours of the curry.
Marwai Da Adde is now ready to serve.