In a deep saucepan, add the sesame oil. When hot, add the mustard seeds. When it starts popping, add the remaining dals and fenugreek to it.
Keep the heat on low-medium. You do not want to burn the dals or fenugreek, which turns bitter when even slightly burnt.
When the dal gets reddish brown, add the onions.
Add salt and asafoetida and give it a stir.
Add the water along with curry leaves. (You can also add it along with the onions instead of now or add half the water then and half now.)
Increase the heat to medium-high.
When the water starts boiling, add the tamarind paste.
After around 2 minutes, add the sambar powder.
Stir until combined and continue cooking until the raw smell of tamarind is replaced with a tangy aroma.
You can at this point continue cooking (simmering) for another 6-7 minutes until the mixture thickens or (after reducing heat to low-medium) mix 1 tsp of rice flour along with 1 tbsp cold water into a paste and add it to the stew.