Dry-fry the cumin seeds and chilli flakes for 1 min, or until they start to jump around the pan and release their aromas. Grind as smooth as possible.
Place the oil, carrot, lentils, stock and milk in a large saucepan and bring to the boil. Add the ground spices and simmer for 15 mins until the lentils have swollen and softened.
Whizz the soup with a stick blender or in a food processor until smooth (or leave it chunky if you prefer). Season to taste and finish with a dollop of yogurt and a sprinkling of the reserved toasted spices.
Perfect served with warm naan breads.