Wash, and chop Sweet potatoes into thickish roundels (any shape works fine as long as they are evenly cut).
Pressure cook for 1-2 whistles until done but not mushy (alternately, you can boil them in a large pan with lots of water and its fork tender).
Meanwhile, Soak the tamarind in 2 cups of HOT water for 15/ 20 mins and extract the pulp.
In a deep bottomed pan, heat the sesame oil, splutter the tempering ingredients on a low flame, once the toor dal slightly browns, add the spice powder and turmeric and fry for 15-20 seconds.
Now add the tamarind pulp (strained of all dirt), add the salt and let it simmer for 10-12 mins.
Then add the cooked sweet potato, jaggery and let it cook for further 1-2 mins
If at this stage, the gravy doesnt thicken, add 2 tsp of rice flour in 4 tsp of water and mix well. Add this slurry to the simmering gravy.
Let it thicken till a spoon appears glossy and oil separates.
Serve hot / warm with above suggested dishes. Tastes great especially with rice and pongal too :)