How to Make It
Heat 1/4 cup of the oil in a skillet until almost smoking. Add the bread cubes and fry over moderately high heat, stirring, until well browned, about 5 minutes. Transfer to paper towels to drain.
Heat the remaining 2 tablespoons of oil in a large saucepan. Add the onion and cook over moderate heat, stirring, until softened, about 5 minutes. Add the potatoes and chicken stock and bring to a boil. Reduce the heat to low, cover and simmer until the potatoes are tender, about 15 minutes.
Stir in the watercress and sorrel, cover and simmer over low heat until the greens are wilted, about 5 minutes. Working in batches, puree the soup in a blender. Return the soup to the saucepan and rewarm over moderately high heat; season with salt and pepper. Ladle the soup into bowls and garnish with the croutons.