Blend coconut, 1/4 of the onion, and tomato until a paste is formed (add a little water to loosen the mixture if needed).
Add the sambar powder and blend again, until everything is well incorporated.
In a stockpot over medium heat, sauté the mustard seeds until they start to pop.
Add the cumin, asafetida, and curry leaves and stir together for a minute or two.
Add the diced onion and vegetables and sauté until halfway cooked/fork tender.
Add the blended masala and 1 cup of water.
Bring to a boil, then add the cooked daal and salt.
Add the rest of the water and bring to a boil again.
Add cilantro and let cool slightly before serving.