Heat oil in a pan or a kadai.
Add chopped onion and saute till golden brown.
Cool it and grind it finely in a mixer grinder along with tamarind, chilly powder and salt.
Heat oil in the same pan.
Add asafoetida.
When the smell comes, add mustard seeds.
When it splutters, add red chillies and curry leaves.
Add the onion paste and stir.
Boil and check the level of salt.
Switch off the fire and add the piece of jaggery and stir in.
:- Serve hot with idli or dosa.