Melt the butter in a large pan, stir in the flour and let the mixture simmer for about 5 minutes on a low heat. stir occasionally. Pour it out already stirring with a whisk with small dashes at the same time add the stock and stir until the mixture is smooth before pouring the next amount of stock in the pan. Let the soup simmer for another 1 to 2 minutes (or turn off the heat and heat up the soup just before serving).

Meanwhile, mix the semi-stiff cream with the egg yolks. Stir just before serving first the mustard through the soup and then the cream yolk mixture. This soup needs no decoration, but for the color a tuft of chervil or parsley can do little harm ...