Heat butter in a large saucepan until bubbling. Add leek and sauté for 5 minutes or until softened. Add the potatoes and chicken stock, cover and simmer for 15 minutes or until the potatoes are tender.
Stir Leafy Patch Watercress into the soup and cook for 5 minutes or until wilted and tender. Season to taste with salt and pepper. Transfer soup to a blender and blend for 2-3 minutes or until smooth and green.
Pour back into the saucepan and reheat gently. Serve with a dollop of sour cream and grinding of black pepper.