Soak the tamarind in 2 cups of water for 30 to 45 minutes and extract the juice. If you are using paste, mix it in 2 cups of water. For this measure, you can use 1.5 to 2 tbsp of tamarind paste.
Heat a kadai or heavy-bottom vessel and add the sesame oil. When the oil is hot, add the mustard seeds, fenugreek seeds, curry leaves, and asafetida. Allow the mustard seeds to splutter.
Then add the chopped ginger and green chili. Mix well and saute for two to minutes.
Then add the tamarind juice, followed by turmeric powder and salt.
Let it simmer for 10 minutes over medium-low heat and then add the jaggery. Mix thoroughly and simmer it again 8 to 10 minutes. The mixture will slowly thicken, and the oil will float on top. That's the right stage to turn off the heat.
Let it cool, before storing it in an air-tight container. You can serve this as a side for yogurt rice, idli, dosa and also mix it with rice and relish.
If you want thin consistency, after adding jaggery, add half more cup of water and simmer it for just 5 minutes. If you want a thick consistency, extract the tamarind juice with 1 cup of water instead of two cups and also after adding jaggery simmer it for 3 to 4 minutes or until the jaggery melts.
Adjust the ingredients, including salt and jaggery according to your spice preference.
Allow it to cool completely before refrigerating.
Instead of sesame oil, you can use peanut oil or any other neutral oil. I have never tried this with coconut oil or vegetable oil.
You can also reduce the spice level by removing the seeds from green chilies before chopping.