Clean husked green gram in water few times. Drain the water. Pressure cook with 1/2 cup of water.
Chop tomatoes. Peel and chop potatoes. Add 1 tsp oil in a sauce pan and add vegetables. Add 1/4 cup of water and let it cook till vegetables are soft and cooked.
Grind coconut with green chillies and cumin seeds with little water to make a paste.
Add Cooked lentils and coconut paste in the sauce pan. Mix well and adjust the consistency by adding a little water. This curry is thick. So, do not add too much of water.
Add add turmeric powder, mix and let the curry simmer for few minutes.
Add salt to taste and mix well and remove from flame.
Heat oil in a small frying pan. Add mustard seeds and white lentil.
Once mustard seeds crackle, add red chillies and curry leaves.
Add this tempering over the curry and serve.