In a heavy saucepan cook the onion, the carrot, the mustard seeds, and salt and pepper to taste in the butter over moderate heat, stirring, until the onion is soft, add the broccoli, the broth, and the water, and simmer the mixture, covered, for 15 to 20 minutes, or until the broccoli is very tender. In a blender purée the soup in batches until it is smooth, transferring it as it is puréed to another heavy saucepan. Whisk in the lemon juice and salt and pepper to taste, heat the soup over moderately low heat, and whisk in the sour cream (do not let the soup boil).
