Heat a large pan over a gentle heat and soften the onion in the oil for about 10 minutes. Add the garlic and ginger and cook for a minute before adding the spices. Cook for a minute or two until the seeds sizzle.
Add the lentils and cook for another minute, then pour in the stock and 500ml water. Season and bring to a boil, then simmer gently for 15 minutes, stirring occasionally, until the lentils are tender.
Pour half of the soup into a blender and process until smooth then return to the pan. Add the coriander stalks and spring greens. Add a little more water if needed then cover and bring to a simmer for a few minutes until the greens are cooked.
Add the lime juice and check seasoning. Serve with a dollop of the yoghurt and the coriander leaves.