Heat oil in a cooking pan. Once the oil gets heated up, add the mustard seeds and let them pop. Throw in the asafoetida, dry red chillies, cumin seeds, fenugreek, toor dhal and cook for couple of minutes. Add the curry leaves and onions. Cook for further 3- 5 minutes on medium high flame. Sprinkle the turmeric powder, rasam podi, little chilli powder and salt. Cook till the raw smell disappears. Pour in the tamarind juice and let it boil. Add 50ml water to dilute the soup. Cook covered for 15min. Serve hot.