Finnan Haddie and Watercress Soup

Preparation

1. In a kettle cook the potato, peeled and diced, and the onion in 4 tablespoons of the butter over moderately low heat, stirring occasionally, until the onion is softened. While the vegetables are cooking, put the finnan haddie in a deep skillet just large enough to hold it, add the milk, the remaining 2 tablespoons butter, and 2 cups water, and bring the liquid to a simmer over moderate heat. Turn the finnan haddie and cook it, covered, at a bare simmer for 3 minutes. Transfer the finnan haddie with a slotted spoon to a plate. To the kettle add the fish poaching liquid and 2 cups water and simmer the mixture, covered, until the potatoes are soft. Stir in the watercress and simmer the soup, uncovered, for 3 minutes. In a blender purée the soup in batches until it is smooth, return it to the kettle, and stir in the lemon juice and salt and pepper to taste. The soup and the finnan haddie may be prepared 1 day in advance and kept separately, covered and chilled. Heat the soup, stirring, until it is hot, ladle it into heated bowls, and garnish each serving with some of the finnan haddie, flaked.