Chicken stock may also be used as the base.
Melt butter in soup kettle. Saute shallots, celery, and carrot until tender; do not brown. Add stock, bring mixture to a boil, immediately reduce heat, and simmer gently 20 minutes, partially covered. Press mixture through food mill. Return to kettle and reheat. In small bowl beat together egg yolk and cream. Add ½ cup hot soup liquid to egg mixture, blend with whisk, and pour back into kettle. Reheat, stirring slowly, but do not boil. Season with salt and pepper. Garnish with chopped summer savory or chives. (Also good cold.)