Soak tamarind and extract the juice. Cut bitter orange into small pieces. Heat oil and splutter the mustard seeds. Then add asafoetida. Then add the green chillies and the cut bitter orange pieces and fry until the bitter orange is cooked fully. Add the tamarind juice and salt to the frying mixture, and allow it to boil until the raw smell of tamarind disappears. Add jaggery and heat in low flame until the mixture becomes a thick paste.