Heat the butter and blend in the flour. Stir in the stock, beating with a wire whisk. When the mixture is thickened and smooth, add the cream. Add salt to taste and the nutmeg. Simmer, stirring frequently, for ten minutes.
Remove from the heat. Add a little of the hot mixture to the beaten egg yolk, then return to the sauce in the pan. Reheat for one or two minutes, but do not let the sauce boil after the egg is added.