Wash and wipe ginger and green chillies. Peel ginger and chop into very fine pieces. Deseed and chop green chillies into very fine pieces. Heat oil in a deep bottomed heavy kadhai, add asafoetida powder and mustard, when it splutters, add chopped chillies and ginger. Fry well till they are golden brown. Add some water and tamarind paste. Add salt to taste and jaggery and let it simmer till oil floats on top.