For lamb or fish.
Melt the butter and over low heat stir in the flour. Gradually add the chicken stock, then the mustard, Worcestershire sauce and lemon juice, and stir until thick and smooth. Turn off the heat and add the egg yolk, cream, and capers, and again stir until smooth. Serve at once; if the sauce is to be reheated later, be sure that it does not boil.