Heat the oil for deep-frying in a kadhai or deep, heavy-based pan to 180°C / 350’F, or until a cube of bread browns in 30 seconds.
Working in batches, add the okra pieces to the hot oil and deep-fry for about 2-3 minutes, or until crisp. Remove with a slotted spoon and drain on kitchen paper (paper towels). Reserve the oil.
Put the cashews and coconut into a small blender or food processor and process, adding a little water if necessary, to make a paste.
Heat the reserved oil in a large, heavy-based pan over medium heat, add the cumin seeds, mustard seeds, urad dal, dried red chillies and curry leaves and stir-fry for about 1 minute, or until the seeds start to splutter.
Add the onion and stir-fry for about 5-7 minutes, or until golden brown. Now add the tomato, stir and add the chilli powder, turmeric, and coriander, and season with salt. Stir-fry for about 2-3 minutes, or until the oil separates out.
Reduce the heat to low, add the cashew and coconut paste and fry for a further 2 minutes.
Remove the pan from the heat, stir in the yoghurt, then pour in 400ml / 14fl oz (1 2/3 cups) water, return to the heat and bring to the boil.
Now add the deep-fried okra, reduce the heat and simmer for about 3-4 minutes, or until coated in the sauce. Adjust the seasoning, if necessary, before serving.