Wash and air-dry the okra. Trim the ends and cut each okra into maximum of 3 pieces. Set aside.
Take a kadai or any heavy pan with lid in which you want to cook. Set it on stove. Add 2 tablespoon of oil and add the cut okra. Sprinkle ¼ teaspoon salt. Let okra brown for 3-4 minutes on medium heat. Don't stir much. Let the skin char a bit. Okra will turn deep green in color. Take it out in a plate.
Add the remaining mustard oil to the kadai and heat up till a bit smoky. Temper the oil with mustard, fennel and cumin seeds. As soon as they crackle, add the onions and green chilies.
Let the onions cook in oil till they are a nice shade of golden brown. Add the garlic next and saute for a minute till you smell a nice aroma.
Add the coriander, turmeric and red chili powder next and saute for a minute or so.
Next add the gram flour, salt and mix very well. The gram flour will coat the onions but make sure its not lumpy. Roast it slightly for 3-4 minutes stirring continuously but dont let it change too much color.
Sprinkle the water on the gram flour, it will steam. Immediately start mixing to make sure that the besan absorbs it and becomes slightly wet. We need to cook the besan now. Lower the heat to low and cover the kadai with lid. Let cook for 5-7 minutes. Check once it between, if th ebesan is getting dry, add a tiny splash of water. Dont let the besan dry out too much right now. It should be moist but not raw.
Once the raw taste of besan goes away, open the lid and add the okra now. With gentle hands, mix with the spiced besan. Cover the kadai again with a lid. Let cook for not more than 2-3 minutes. Remove the lid and you will see that okra is much softer, cook for another minute if you see that its not tender .Once okra is cooked properly, its slimy texture goes away.
Finish the with amchoor and garam masala. Sprinkle some chopped cilantro. Mix well and serve warm. As the dish sits, it is but natural that the besan will dry up and turn into small granules.