Wash both varieties of rice and soak them in water for 4–5 hours. Drain, then grind with the coconut and salt to a semi-smooth paste, adding water as needed.
Transfer the rice paste to a thick-bottomed pan. Cook over medium heat, stirring, until the extra moisture evaporates and the mixture forms a thick dough.
When the dough is cool enough to handle, grease your hands with a little coconut oil. Shape it into balls and press your thumb into the middle of each one to make a dent.
Steam the pundi for 30 minutes, then set aside.
Roast the coriander seeds, cumin seeds, fenugreek seeds, mustard seeds and peppercorns. Roast the Byadagi chillies in 1 tsp coconut oil.
Grind the roasted spices and chillies with the coconut, turmeric, tamarind, garlic and chopped half-onion to a smooth paste, using a little water.
Heat 2 tbsp coconut oil in a large saucepan. Add the large chopped onion and fry until golden brown.
Add the ground coconut masala and water to reach your preferred curry consistency. Add salt sparingly and bring to a boil, remembering that the clams and their cooking liquid will add saltiness.
Add the steamed pundi and simmer for a few minutes.
Wash the clams well, brushing away any sand from the shells. In a separate pan, cook them with a little water until their shells open. Discard any that remain closed after cooking.
Add the cooked clams and their cooking liquid to the pundi curry. Bring it back to a boil, then switch off the heat.
Let the pundi take in the curry briefly, then serve.
