Fill and boil your kettle. Trim the roots and top 3cm off the leek, then halve it and rinse out any grit. Finely chop the leek. Peel the potato and dice it.
Put a large pan on a medium heat and add the butter to the pan. Swirl it round to melt the butter, than add the leek and potato. Season with a pinch of salt and pepper and pop a lid on the pan. keep the heat at medium-low and fry, stirring every few mins, for 10 mins till the veg have softened.
While the veg fry, separate the lettuce leaves and give them a good wash. Roughly tear them into smaller pieces.
Add 2 tsp ground coriander to the veg and fry, stirring, for 1 min. Then pour in 800ml hot water from the kettle. Bring the pan back to the boil cook for 5 mins. The potatoes should be soft. Add the lettuce and sorrel to the pan, without the lid on (this helps keep the soup green) and simmer for 2 mins to just wilt the lettuce.
Take the soup pan off the heat and use a stick blender to blitz the soup till its smooth, or ladle the soup into a blender and blitz till smooth. Taste and add a pinch of salt and pepper if you think it needs it. Ladle the soup into warm bowls and top with the pine nuts to serve.