Break three stalks of celery in one-inch pieces and pound in a mortar. Cook in double boiler with one slice of onion and three cups of milk for twenty minutes. Remove onion, heat two tablespoons of butter, add two tablespoons of flour, one-fourth teaspoon of pepper, one teaspoon of salt; first two-thirds of a cup, and gradually the rest of the celery broth, add one cup of cream; cook until smooth and serve at once.