Bring 4 cups of water to a boil.
Add lentils. Cook until tender, about 10 minutes.
In a wok or large skillet, heat oil.
Add mustard seed, curry leaves and chiles. After they stop sizzling and popping, add onion. Saute until softened.
Add cooked lentils, turmeric, salt and coriander. Stir to combine.
In a small bowl, whisk together the yogurt and the besan.
Add this to the lentils and simmer for 5 minutes.