Heat the oil and add the asafoetida, mustard and cumin seeds. When they sputter, add the curry leaves.
Add the tamarind juice and turmeric bring to a boil. Cook for 5 minutes, then add the sambar powder. Stir well.
Add the veggies and more water, if needed. Cook until veggies are fairly tender, about 5 more minutes.
Add the lentils, or dal, and stir together well. Add salt to taste.
Bring everything to a boil. Lower heat to a simmer, and cook for another 5 minutes.
Serve hot over rice.