Soak channas in water overnight. Next morning, boil them with a pinch of soda and salt until tender and dry. Heat 4 tablespoons of ghee and put in ginger and onions and fry till the mixture turns limp and almond coloured. Add tomatoes and spices and cook till the ghee starts separating. Mash with a wooden spoon to a smooth paste then put in 1 cup water and jaggery and channas. Mix well and bring to a boil, reduce heat and put in chillies and potatoes and cook till the chillies are done. In the meantime soak tamarind in 4 tablespoons water till soft, squeeze out the pulp and put into the channas. Mix well and then cook until the gravy turns quite thick. Remove from fire and serve hot garnished with onion rings, minced chillies, coriander, mint and lime wedges.