Take of the coarser parts of celery as much as will make two heads, wash
and cut in pieces, put in a saucepan with half an onion cut in slices
and cover with boiling water. Cook until tender and press through a
sieve with the water in which it was boiled. Make a roux of butter and
flour as in other cream soups, add the puree to it and as much boiling
milk as will make it the proper consistency. Season with salt and
pepper, and finish with a beaten egg yolk and two tablespoonfuls of
cream, adding this after the soup has been removed from the fire.