This is the one I use, it is for domestic leeks so you may want to reduce the amount of wild ones.
3 cups sliced leeks, white and tender green parts
3 cups peeled and roughly chopped baking potatoes, like russets
6 cups water
11/2 teaspoons salt
1/2 cup heavy cream
1/2 cup creme fraiche or sour cream
1/3 cup minced chives or parsley
In a large heavy saucepan, bring the leeks, potatoes, water, and salt to the boil over high heat. Cover partially, reduce heat, and simmer for 20 to 30 minutes until the vegetables are tender. Correct the seasoning, to taste, and puree with a handheld immersion blender, or in batches in a food processor.
Whisk in the cream and reheat before serving. Top each serving with a dollop of creme fraiche and sprinkling of fresh chives.
To Make creme fraiche- mix two table spoons of buttermilk into a cup of heavy cream, stir well. Cover container with a clean cloth and let sit on countertop for 20 to 24 hours until the mixture thickens. Then cover and put in the fridge. While you can use sour cream, creme fraiche is better and easy to make.