Wash and carefully dry the okra. Trim the tops, cut in half lengthways, then cut each piece into 3 chunks (each about 2cm long). Spread out on a baking tray and leave uncovered to oxidise and dry for about 24 hours.
Crush the chillies and garlic together using a pestle and mortar (or a blender), to make a fine masala paste.
Peel the potatoes and cut into chunks roughly 1 x 4cm.
Heat the oil in a large frying pan for about 1 minute over a medium heat, then add the fenugreek and mustard seeds. When the mustard seeds start to pop, stir in the asafoetida and potatoes, ensuring that the potatoes are well coated in oil. Increase the heat to high and fry for 1 minute, then stir in the okra and return the heat to medium.
Gently stir in the salt and turmeric, then cover the pan and leave to cook for three minutes. Add the masala paste, ground coriander, ground cumin and chilli powder and stir gently to mix, being careful not to break the delicate okra. Cover and leave to cook for 5 minutes, or until the potatoes are cooked through. Stir once more, then remove the pan from the heat and leave to rest, covered, for about 5 minutes to allow the flavours to infuse.
Reheat over a low heat until piping hot, then sprinkle with the chopped coriander and serve.