1. Chop the onions and tomatoes.
2. Heat oil in a pan.
3. Add the mustard seed when they stop spluttering add urad dhal and channa dhal, followed by curry leaves.
4. Add the onions, saute the onions until they turn translucent.
5. Add the tomatoes along with turmeric and gosthu powder, when the tomatoes get cooked well add the tamarind extract and mix them well.
6. Add sufficient water along with salt.
7. Allow it to cook in low flame for about 10 - 15 mins.
8. Onion gotsu is ready to be served.

Courtesy: Veggie Paradise