For the paste, put the fresh coconut, ginger, cumin seeds and chilli in a blender, add 200 ml water and blitz to a smooth paste.
Put the oil in a heavy-based saucepan on a medium heat. Add the mustard seeds, dried chilli and curry leaves. After 15 or so seconds, when they start to splutter and pop, add the chopped onions and fry, stirring, for 7 to 8 minutes, until they start to soften.
Add the turmeric and the chopped carrots and potatoes, fry, stirring, for just a few seconds, then pour in 200 ml water, cover and bring to a simmer. Leave to cook gently for 10 minutes, then add the chopped green beans and cook for another 8 minutes, until all the vegetables are tender.
Add the coconut paste and the ground black pepper, season with salt to taste, then stir and simmer for another 2 minutes. Whisk the yoghurt, then stir it into the pan and simmer for a minute more. Garnish with coriander and serve at once with rice or paratha.