- Cook bones and vegetables in 1½ quarts water for about 2 hours.
- Keep skimming the broth to remove scum. Strain.
- Add enough water to bring the amount to 1½ quarts again.
- Add barley to the broth and cook until it is soft.
- Make a roux with butter and flour.
- Cook the roux over low heat stirring constantly but do not let it become dark.
- This should be a light roux.
- Stir ⅓ cup cold water and ⅓ cup cold milk into the roux and whip until smooth.
- Pour into soup and simmer it for 10 minutes.
- Puree the soup in a blender or through a sieve.
- Adjust the salt to taste.
- Mix cream with egg yolk and put it in a soup tureen.
- Just before serving pour hot creamed soup over the yolk and cream mixture.