Step 1Tear the baguette into rough pieces and put the pieces in a bowl with water to cover. Soak for at least 15 minutes.
Step 2Coarsely chop the cucumbers and place them in a blender in batches. Chop most of the sorrel leaves, reserving two for garnish. Add the sorrel leaves to the blender with the garlic, salt and yogurt, and puree until smooth.
Step 3Remove the bread from the water and squeeze dry. Add the bread to the blender and puree the mixture until perfectly smooth. Pour it through a strainer into a deep bowl, discarding any bits of bread caught in the strainer. The soup should be slightly thickened, about the texture of heavy cream. Cover the bowl tightly and refrigerate for at least 1 hour.
Step 4To serve, season the soup to taste with more salt if necessary and ladle it into wide bowls. Use a large spoon to swirl in a streak of yogurt. Thinly slice the reserved sorrel leaves and scatter a few slices across the top of the soup.