Soak tamarind in 1/4 cup water and take out thick juice. One or two tablespoons of thick juice will do.
Wash potatos and peel the skin. Cut it into bite size pieces. Cut brinjal also to the same size. Put both the potato and brinjal pieces in a broad vessel. Add red chilli powder, coriander powder, turmeric powder, salt and tamarind juice. Mix well and marinate for ten to fifteen minutes.
In a kadai put the oil and when it is hot add mustard. When it pops up add urad dhal, asafoetida powder and fry till urad dhal turn light brown. Add chopped onion and curry leaves and fry for few seconds. Then add the marinated potato and brinjal pieces. Stir well. Sprinkle half cup water and once again mix it well. Cover with a lid and cook on low flame stiring now and then till the potato is soft.