In a large dutch oven or pot, combine the lentils, coarsely chopped onion, tomatoes, cayenne, ground cumin, coriander, turmeric, and salt. Add 6 cups of water and bring to a simmer. Cook uncovered for about 30 minutes, or until the lentils begin to soften.
Tadka
In a small skillet on medium heat, add olive oil. Once shimmering, add the cumin seeds and the mustard seeds.
As soon as the seeds begin to sputter, add the chopped onion and the curry leaves and saute for about 1-2 minutes, Stir constantly to prevent the leaves from burning.
Add the Tadka to the lentils. Also stir in the coconut milk. Cook for 10-15 minutes until heated through. Add salt to taste if needed and serve with basmati rice!