To make the ground masala, fry the coriander seds, chana dal and red chilies in the oil until lightly golden and then grind into a fairly smooth paste with the coconut milk, adding a little water if necessary.
Cook all the vegetables until they are almost tender (I zapped them in the microwave with about a cupful of water for about 8 minutes)
Put the vegetables into a large saucepan over medium heat.
Add the tamarind water and cook another 2-3 minutes
Now add the cooked dal, sambar powder, and the ground masala paste. Give it all a good stir and allow it to come to a boil.
Add salt to taste. If the kuzhambu is too thick, add some water.
Let the kuzhambu or sambar cook on a low heat for about 10 minutes, turn off heat.
Heat the oil for tempering and add the remaining tempering ingredients.
When the mustard seeds crackle, take the tempering off the heat and add it to the sambar.
Garnish the sambar with coriander leaves. Serve hot with rice and poppadum.