Cook the celery in salted water, covered, until tender but still crisp. Drain, reserving 1 cup of the liquid. Melt half the butter, stir in the flour, and gradually the reserved celery liquid and cream. Cook over low heat until mixture bubbles and thickens. Simmer for 5 minutes longer. Add the celery, mushrooms, and chestnuts, and season to taste. Turn the mixture into a serving dish, and sprinkle bread cubes sautéed in the remaining butter over the top.