Cook celery in the boiling water until soft. Drain and save liquid. Put celery through a sieve.
Melt the butter, stir in flour, add celery, 2 cups of the water in which it was cooked and stock, stirring until smooth. Add salt and pepper.
Beat egg yolk and add with the peas. Boil 3 minutes. Turn off heat. Add sherry and stir well to blend.