1. Put a large saucepan over a medium heat then add the cumin seeds. Cook for 2 minutes, or until toasty and fragrant. Set ½ tsp aside for later.
2. Add the butter to the pan, let it melt and sizzle around the cumin, then stir in the leeks, ginger, coriander stalks and most of the chilli. Cover and let the flavours sweat together for 5 minutes.
3. Stir in the potato and lentils, then the stock and milk. Bring to a boil then simmer for 15 minutes, or until the lentils are soft.
4. Blitz the soup in the saucepan using a stick blender, then season to taste with salt and pepper. Ladle into bowls, add a swirl of crème fraiche or yogurt, then scatter with the reserved chilli, toasted cumin and coriander leaves. Enjoy with warm naan bread.