Soak the mung beans overnight in large bowl of water. Drain.
Heat the oil in a large pot. Add the cumin seeds and cook for about 1 minute, until they’re fragment. Add the garlic & ginger, then stir-fry 3 minutes until they start to brown. Add the spices, stir for 30 seconds, then add the tomato purée and stir again,until well coated.
Add the tinned tomatoes, water and mung beans. Increase the heat to high and bring to the boil, then reduce to low, cover, and simmer for 30 minutes or so, until the beans are cooked through.
Add the coconut milk and increase the heat to medium-high. Cook for another 10 minutes or so, until the sauce has thickened.
Turn off the heat, stir in the lime juice and coriander leaves. Taste and season to your liking.
Serve hot.