In a medium saucepan, add butter and cook over medium heat until melted. Add the leek and onion and cook for 5 minutes, or until soft. Add the stock and bring to a boil. Reduce heat until the soup simmers. Add peas and cook for 5 minutes.

Remove the pan from heat and combine the soup and parsley in a food processor. Purée the soup until smooth. Return soup to a clean pan and bring to a simmer. Add crème fraîche, season with salt and pepper, and serve.