Soak thuvar dhal and rice in water for about 1 to 2 hours.
Soak separately tamarind and salt in one cup of water.
Wash the dhal and drain out the water completely. Put it in a mixie and grind to a thick coarse paste (like vadai dough). Add ½ teaspoon salt, one teaspoon sambar powder, a pinch of turmeric powder, chopped onions, one tablespoon coconut gratings and few curry leaves chopped to the paste. Mix well. Make a small lemon size balls
Squeeze out the tamarind and extract the juice. Add more water and again take out the remaining juice. The tamarind juice should be of 3 cups. Add sambar powder and turmeric powder to the tamarind juice. Bring to boil. When it starts boiling, add two to three dhal balls and reduce to heat to medium. Allow to boil. When it starts boiling again, then add three more balls. Again wait for the Kuzhambu to start boiling. Add three more balls and finish all the balls like this. Once you finished all the dhal urundais, gently turn them with a spoon. Keep the flame to low.
Grind two tablespoon coconut gratings to a fine paste and add to the boiling kuzhambu.
Season it with mustard, chopped onion and curry leaves.
Note: Some people steam the dhal balls and add to the kuzhambu. You can also do it. However, when you prepare kuzhambu as above, the dhal balls cook in the kuzhambu itself and all the spices will go inside the dhal and tastes good.