Heat the oil in a hot wok or cast-iron pan on medium-high heat and add the cumin seeds. As soon as they begin to bloom, add the sliced onions.
Fry the onion until soft and slightly golden. Turn the heat to high and add the okra that you have sliced lengthwise into four thin quarters each.
Fry the okra with the onion on high heat, continuously stirring. The okra will become fragrant and some sliminess will start sticking to the bottom. Don't worry about this--the chickpea flour will take care of this later.
When the okra has become soft and turned a slightly darker colour, after about 5 minutes, turn the head down to medium high and add the ground coriander, salt, cayenne and turmeric. Stir again.
Fry for another 2 minutes or so, incorporating the spices into the okra and onions evenly. Now, sprinkle the chickpea flour over the okra and stir very well.
Stir the mixture very well. You don't want to be eating any raw chickpea flour. The flour will turn a lovely golden brown colour as it cooks and the okra will have turned an almost emerald dark green. The flour will stick to the okra slime on the bottom and on the sides too. Scrape this with a wooden spoon and incorporate it into the mixture. It will lend the dish a wonderful earthy crispness.
Once the chickpea flour looks cooked, after about 5 minutes, sprinkle over the dried mango powder and nigella seeds and stir. Dried mango powder tends to clump together in the box, so separate the grains with your fingers as you sprinkle. Cook for another minute or so, then turn off the heat.
Serve with warm rotis and cucumber yoghurt as an entree or with basmati rice and dal as a side dish.