Heat half the oil in a large, heavy-based pan. Add the okra and stir-fry for 1 minute.
Put the ground coriander, chilli powder and turmeric in a small bowl, add 2 tablespoons water and, stir together.
Heat the remaining oil in a heavy-based pan over medium heat, add the onion and a pinch of salt, and stir-fry for about 2 minutes, or until the onion is translucent. Add the garlic paste, and continue frying for a further 2 minutes, or until the onion is lightly golden.
Add the green chillies and stir-fry for about 30 seconds, then add the coriander mixture and stir-fry for about 2-3 minutes, or until all the moisture has evaporated. Add the pureed tomatoes and stir-fry for about 2-3 minutes, or until the oil has separated out and rises to the surface.
Add the tamarind extract and stir-fry for 1 minute, then pour in 250ml / 8fl oz (1 cup) water, add the sugar, and season with salt. Bring to the boil, then reduce the heat to low and simmer for about 2-3 minutes.
Now add the okra and simmer for about 5 minutes, while you make the tempering.
Heat the oil for the tempering in a small frying pan (skillet) over medium heat. Add the mustard seeds and stir-fry for about 1 minute, or until they start to splutter. Add the fenugreek seeds and curry leaves and stir-fry for 1-2 minutes, or until the leaves stop spluttering.
Pour over the simmering okra, then remove from the heat and adjust the seasoning, if necessary.