Heat a large saucepan and dry-fry the cumin seeds and chilli flakes for 1 min, or until they start to jump around the pan and release their aromas. Add the oil and onion, and cook for 5 mins. Stir in the lentils, stock and tomatoes, then bring to the boil. Simmer for 15 mins until the lentils have softened. Whizz the soup with a stick blender or in a food processor until it is a rough purée, pour back into the pan and add the chickpeas. Heat gently, season well and stir in the coriander. Finish with a dollop of yogurt and coriander leaves.