How to Make It
In a large pot, heat the butter over moderately low heat. Add the onions and cook, stirring occasionally, until translucent, about 5 minutes. Add the stock, potato, sorrel and salt and bring to a boil. Reduce the heat and simmer, partially covered, until the potatoes are soft, about 20 minutes.
Meanwhile, combine the smoked salmon, herbs and a pinch of pepper. Top each croûte with a small mound of the salmon mixture.
In a blender, puree the soup. Return to the pot and add the cream and 1/4 teaspoon pepper. Bring to a simmer and serve topped with the croûtes.