Wash okra and dry by patting with a tea towel. Trim the top and bottom tips off. Slice each okra diagonally into 2-4 pieces.
Heat oil in a wok on high. Add cumin seeds, mustard seeds, chilli, garlic and ginger. In a few seconds when it starts to sizzle and the mustard seeds start popping, gently add the tomato and onion. Reduce heat to medium and sauté for a few minutes until onion is caramelised and the tomato is soft. Add ground coriander, cumin, garam masala and turmeric. Sauté for another minute or so till the masala mix is tender and fragrant.
Add the okra and salt. Stir well to mix evenly. Add a quarter cup water to the pan. Cover and cook until water has evaporated.
Remove from heat. Stir in the mango powder and lemon juice. Garnish with coriander and serve hot with steamed rice or naan.
Notes
Dried mango powder is called amchoor and can be bought in Indian grocery stores and online. It is sour and pungent and works well with okra. If you can't find it, you could sub it with a dash of pomegranate molasses or tamarind pulp or even a little sprinkle of sumac.