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. Scoop out about half of the seeds with a spoon and set aside. Add the oil, carrot, lentils, stock and milk to the pan and bring to the boil. Simmer for 15 mins until the lentils have swollen and softened.
Whizz the soup with a hand 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.
Courtesy of the BBC Good Food Guide