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In a blender, combine the onion, turmeric, ginger, garlic and 1 tbsp of the oil together with a large pinch of salt. Heat the remaining oil in a shallow pan. Add the mustard seeds and curry leaves, and cook until they crackle, then add the onion paste and cook until sticky.
Stir through the curry powder, then pour in the coconut milk. Add the cloves, cinnamon and the chilli, and bring to a simmer. Tip in the beans and simmer for 15 mins or until the beans are tender. Squeeze in the lime juice, add the garam masala, take the pan off the heat and stir through the coriander. Serve with rice and rotis, if you like.