Toast the cumin and mustard seeds in a medium sized pan until they start to pop. Remove the pan from the heat and leave for one minute to cool slightly, then carefully add 2 tablespoons of water to the pan (it will be super hot and sizzle so take care). Put the pan back on the heat and add the onion and a pinch of salt. Sauté for 5 minutes then add the chilli, ginger and garlic to the pan. Cook for another 2 minutes, adding a litte more water if needed.
Add the chopped tomatoes, chilli powder, coriander powder and ground turmeric to the pan. Stir well to combine and cook for another 2 minutes before stirring in the chopped potatoes and tomato purée. Stir well to coat the potatoes then add enough water to create enough fluid to just cover them. Bring to the boil, then reduce the heat and simmer for 10 minutes until the potatoes are just cooked.
Add the okra to the pan and continue to cook for another 5 minutes or so until it is just soft - try not to over cook it or you will get more slime than you might enjoy!
Season with salt and pepper. Serve garnished with fresh coriander and a sprinkle of garum masala (both optional).