Published March 1, 2010.
Unfortunately, the very thing that makes this rustic soup a snap—the blender—also contributes to its downfall.
Modern versions of this centuries-old recipe fall short, turning out stodgy and gluey, or blending in so much cream and butter that the flavors of leek and potato are lost.
We wanted a creamy soup that still had the clear vegetable flavor of leeks and potatoes.
Virtually all recipes call for simmered potatoes and leeks to be pureed in a blender, but in one notable exception, Julia Child recommends passing the leeks and potatoes through a food mill. Whereas other soups come out gluey and bland, hers has an unprecedented vegetable flavor and a remarkably creamy consistency—with only 2 tablespoons of cream. We realized that the blender is the culprit, which makes sense: When potatoes are overwhipped (as in a blender), they leach too much starch and turn gluey, which many recipes try to remedy with gobs of flavor-dulling cream. We weren’t about to use a food mill for such a simple soup, so we had to find another way to prevent glueyness.
Putting the texture problem on hold, we drew up a basic recipe to evaluate potato types. We began by sweating the white parts of leeks in butter, and then added water and potatoes. Mild russet potatoes broke down well and won the job, but even with the right potato, the leek flavor was barely discernible. Adding more leeks and caramelizing them resulted in off-putting flavors, but one onion ramped up the sweetness just enough. Next, we replaced half the water with chicken broth and added a bay leaf and a sprig of thyme for grassy notes. And we found that simmering the dark green leek tops that we’d been throwing away added potent leek flavor to the broth.
We then turned back to the consistency. We knew that less potato would mean less starch, which in turn would mean less glueyness, so we considerably reduced the quantity of potatoes. The leeks provided plenty of flavor, but this low-starch, low-cream version was too brothy. We decided to try a technique from our Creamless Creamy Tomato Soup: using sandwich bread as a thickener instead of cream. Bingo! Bread, toasted to remove moisture, added body without disturbing flavor. For texture, we garnished the satiny soup with crisp fried leeks.
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