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A French Soup of Leeks
In the provinces they make many savory soups with vegetables which contain all the mineral salts we need so much for our nerves and blood, especially in the spring. Trim and slice a large bundle of leeks, discard the greenest part of the tops and fry the rest in 2 ounces butter, add 2 ounces flour, then a pint of hot milk and a pint of white stock, parsley, salt, and pepper. Cook 40 minutes, add yolk of one egg, and serve with croutons.
A leek velouté in all but name: a butter-and-flour roux loosened with milk and stock, finished with an egg yolk. The instructions are terse but complete, and only a few points need translating.
- The hinge: the egg yolkOnce the yolk goes in, the soup must not boil or it will curdle into threads. Pull the pan off the heat, whisk a ladleful of hot soup into the beaten yolk, then stir that back into the pot and serve at once.
- "A large bundle of leeks"Genuinely imprecise; the recipe gives no count or weight. Something on the order of 1 to 1 1/4 lb of trimmed white and pale green will balance the quart of liquid, but this is an editorial guess, not a figure from the text.
- PintsIf you are working from an American edition, a pint is 2 cups; British pints are 20 fl oz. Either works, but use the same size for both pints, since more liquid against the same roux gives a thinner soup.
- 2 ounces butter, 2 ounces flourWeights, not volumes: 4 tablespoons butter and a scant 1/2 cup flour. Against the quart of milk and stock that is a conventional medium binding, giving a lightly thickened soup rather than a broth.
- "Fry the rest in butter"Sweat rather than fry. Keep the heat low and the pan covered until the leeks are limp and translucent with no colour; browned leeks turn the finished soup grey and slightly bitter.
- White stockUnsalted veal or chicken stock, made without browning the bones. A light chicken stock is the practical substitute; season at the end, since the original salting assumes an unsalted base.
- Cook 40 minutesLonger than the leeks need. The length is probably there to cook out the raw flour; a bare simmer of twenty-five to thirty minutes, stirring the bottom, will do that.
- YieldAbout a quart of finished soup, or four first-course servings. The croutons are not decoration here; a thickened milk soup wants the contrast.
Soupe aux poireaux belongs to the everyday provincial cooking of France rather than to restaurant menus, and this version shows the moment when that home cooking met the classical technique taught in professional kitchens: a leek soup that a farmhouse would have made with water and bread is here built on a roux, white stock and an egg-yolk liaison. The opening remark about "mineral salts" for the nerves and blood is very much of its period. The popular understanding of nutrition was shifting from simple ideas of nourishment toward the mineral and, shortly after, vitamin content of foods, and cookbook writers routinely justified vegetables in those terms. Leeks in particular carried a long reputation as a spring cleansing food, a claim generally traced to folk medicine rather than to any specific finding. The framing follows from that: the reader is being told not just how to make the soup but why the French eat it.