Wash and peel the celery roots, cut them into dice and cook until tender
in as little water as possible, and when nearly done add a little salt.
Make a sauce of two tablespoonfuls of butter and one tablespoonful of
flour cooked together until smooth without browning. Then add a cup of
rich milk, and when this boils turn the celery dice with the water in
which they were boiled into the sauce, season to taste with salt and
pepper. When ready to serve beat one egg yolk with a tablespoonful of
cream and stir carefully into it, remove at once from the fire, pour
into a vegetable dish, sprinkle with a little parsley minced fine, and
serve.