Wipe the chicken and place in the largest pan you have. Add the onion and bay leaf and cover with cold salted water. Cover with a lid and bring slowly to the boil. Simmer for 1 hour then draw off the heat and allow the chicken to cool in the liquid, preferably overnight. When cold discard the chicken skin and bones and cut the flesh into neat pieces. Arrange on a serving plate.
Remove any fat from the chicken stock and measure out
Place the egg yolks in a small basin and blend with the cream. Mix well and stir into the hot sauce. Stir over low heat for a few moments, but on no account allow the sauce to boil. Draw off the heat and recheck flavour. Pour into a basin and allow to cool, stirring occasionally to prevent a skin forming. The sauce will thicken considerably on cooling. Spoon the cold sauce over the chicken several hours in advance of serving. Then cover and keep in a cool place.