Peel and cut potatoes into 5 cm pieces. Soak cashewnuts in hot water. Grate coconut. Extract coconut milk from 3/4 of the coconut. Boil cashewnuts and potato in the second extraction of coconut milk. Grind to a fine paste the remaining coconut, red chillies, coriander, turmeric and cumin. Slice onion and slit green chillies. Slice ginger and garlic. Heat the oil. Add mustard and curry leaves. When seeds splutter, add ground spices and and sliced ingredients. Cook till onions are soft. Add cooked vegetables, lime juice and salt. Simmer for 10 minutes. Add the first extraction of coconut milk and simmer for 5 more minutes. Test for seasoning and remove.