Melt the butter, stir in the flour, heat until it is light yellow. Add the stock, beat with a whisk. Bring the soup to a boil and cook for about 5 minutes. Mix the creme fraiche and egg yolk and add to the soup, heat. Stir in herbs, season.
Before serving, pour toasted bread cubes or baked peas over the soup.
The recipe can be modified with fish broth and dill.