Heat a large saucepan and then add the chilli flakes and cumin seeds. Dry fry for a minute or so until they release their aroma and pop around in the pan. Remove half of the seeds and set aside.
Add the oil to the pan, and once it is hot, stir in the carrots, apple and celery. Fry for about 5 minutes, and then add the lentils, stock and coconut milk. Bring to a simmer and cook for about 15 minutes, covered with a lid, until the carrots and lentils are tender.
Blitz the soup until smooth, and season with salt and pepper. To serve, spoon into bowls, add a spoon of yoghurt on top, scatter with the reserved spices, and add a few coriander leaves. Serve with warm naan bread.