Put the cumin and coriander seeds in a small dry frying pan (skillet) over medium heat and stir-fry for a few minutes until roasted. Set aside.
Heat the oil in a large, heavy-based pan over medium heat, add all the ingredients for the seasoning and fry for 1-2 minutes, or until the mustard seeds start to splutter.
Add the onions and fry for 5-7 minutes, or until golden brown. Add the turmeric, ginger, and garlic and saute for 2 minutes, then add the roasted cumin and coriander seeds and stir well.
Add the tomatoes and cook for 2-3 minutes, or until the tomatoes are well cooked and the oil has separated out and risen to the surface.
Add the okra, stir, then add the coconut and mix well. Add the chopped mint and coriander leaves, green chillies, chilli powder, and season with salt. Stir well, then pour in 500ml / 18fl oz (2 1/4 cups) water and the coconut milk, reduce the heat, and simmer for a few minutes until hot.