Cut each stalk in 4 parts. Cook in salted boiling water until soft. Drain, save water. Cut half celery in small pieces and save for soup tureen. Rub rest through a sieve.
Melt butter, stir in flour, add celery water and part of bouillon; salt to taste. Stir until thickened, add pureÚd celery; add remaining stock; bring to a boil (3-5 minutes). Add beaten yolk mixed with 1 tablespoon water. Add remaining ingredients and pour in tureen over celery.