Take two handfuls of spinach leaves, remove the hard heart, wash them well, chop and cook them with a piece of butter the size of a small egg. When it has melted, add sufficient hot water, allowing a quarter more for reducing, a little chopped parsley, chervil, and tarragon. Season with salt and pepper, bring to the boil, and put in this one cucumber peeled and cut in small pieces. Cook for about forty minutes. Mash all the herbs and vegetables through a sieve (keeping two or three pieces to be cut in small cubes). Add to this purée some of the stock, so as to obtain a fairly thick soup, a little cream, and bind at the last moment with a yolk of egg. Put in also, just before serving, the small cubes of cucumber, fried croûtons, and a little plucked chervil.