Tip the spices into the pan and briefly heat to toast them. Pour in 800ml water and pile in the carrots, leek and garlic. Crumble in the stock cube, then cover and cook over a high heat for 10 mins.
Tip in the lentils with their liquid and most of the chopped coriander, then cover and cook for 5 mins more. Blitz the soup with a hand blender to part-purée it. Serve topped with the remaining coriander and the yogurt, if you like. Will keep in the fridge for 3 days.