Boil the potatoes in their skins for 40 minutes or so, until a knife tip will slide in easily, then peel and cut into 2cm cubes. Soak the dried tamarind in 200ml of boiling water for 10 minutes, then pulp with your fingers and sieve into a small bowl. Crush the ginger using a pestle and mortar (or a blender) to make a fine pulp.
Heat the oil in a large pan for a minute over a medium heat and add the dried red chillies, cumin seeds and mustard seeds. When the mustard seeds start to pop, reduce the heat to low and stir in the asafoetida, tomatoes and jaggery. Stir over a low heat for a minute or so, until the jaggery has dissolved.
Increase the heat to high and stir in the tamarind water, crushed ginger, chilli powder, ground coriander, turmeric, salt, half the fresh coriander and 400ml boiling water. Cover and cook for 10 minutes to bring the spices together and intensify the flavours.
Add the garam masala and stir well – this is a strong flavour and needs to be thoroughly mixed in before you add the dish's main ingredient. Stir in the potatoes gently to avoid breaking them up, then remove the pan from the ehat, sprinkle with the remaining chopped coriander and leave to rest, covered, for at least 10 minutes to allow the flavours to infuse.
Reheat over a medium heat and serve with puri (unleavened, deep fried indian bread) and kakadhi raitu (a cool cucumber and yoghurt dip).