Soak tamarind in 1/2 cup warm water for 15 minutes and extract thick juice. You can use 3 tablespoons ready made tamarind pulp too.
Heat oil in a thick pan. Temper with mustard seeds, methi seeds and curry leaves.
Add peeled small onions and saute well for 2 minutes. Add the sundried vathal and mix well.
Add turmeric powder, sambar powder or vatha kuzhambu powder and mix very well.
Add the tamarind pulp and mix well. Bring to a boil.
Add 2 cups water. Season with salt. Let the kuzhambu boil for 7-8 minutes. Then simmer for 6-7 minutes. The kuzhambu will thicken well. Add jaggery and mix well.
Once the gravy reaches the desired consistency, add 2 tablespoons gingely oil and remove from flame. Mix well. Serve vathal kuzhambu hot with steamed rice.