Soak the tamarind in 1 cup hot water water and extract the juice
Soak the dals in water for 1-2 hours. After 2 hours, take small quantity of soaked dals with the chillies,salt and hing in a mixer.
Once the chilies are crushed in the mixer, add the remaining dals and grind it once. The dals should not be ground finely in the mixer, they can be coarse. Keep this aside.
Take the ground dal coarse paste in a bowl, add turmeric powder and curry leaves. Mix well
In a pan, pour the oil and add mustard seeds when the oil is hot. When the mustard seeds begins to sputter add fenugreek seeds, channa dal and fry it till it becomes light golden. Add the curry leaves and saute it for a second.
Now add sambar powder and fry it for 30 seconds in medium flame. Add the tamarind extract in the pan along with asafoetida and salt. Normally for a vathakuzhambu, we add a thick extract of tamarind juice, but for paruppu urundai kuzhambu, the tamarind extract should be in a thin consistency. Add 1 more cup of water and dilute the tamarind extract.
Allow the tamarind juice to boil till the raw smell goes off. The reason for diluting the tamarind juice is that the lentil balls are cooked directly in the tamarind extract which will make the balls dissolve a little and that will make the kuzhambu thick atlast.
Make small balls out of the dal paste and keep it ready. Now take 1 teaspoon of rice flour in a plate. Coat each dal dumpling with rice flour.
Gently add the rice flour coated dal ball to the boiling tamarind extract. Repeat the same for the remaining dal paste. Allow the kuzhambu mixture to boil till the parupu urundai are cooked completely in the kuzhambu and do not stir the kuzhambu for the first 1-2 minutes else it will make the balls break in tamarind extract.
After 5 minutes gently stir the kuzhambu and allow the lentil balls to cook completely. Test if the lentil balls are cooked by taking a lentil ball and break it with spatula. See if the inner side of the lentil balls are cooked and if so, then we can swtich off the flame.
Now the Parupu urundai kuzhambu is ready to serve with plain rice