In a large saucepan cook the leek, the celery, and the carrot with the bay leaf and the thyme in the butter over moderate heat, stirring, for 5 minutes, or until the vegetables are softened. Add the potatoes, peeled and sliced thin, and the broth and simmer the mixture, covered, for 15 minutes, or until the potatoes are very tender. Discard the bay leaf and in a blender puree the soup in batches. Transfer the puree to the cleaned pan and stir in the half-and-half. Heat the soup over low heat, whisk in the Stilton, whisking until the cheese is melted and the soup is smooth, and whisk in the Port and salt and pepper to taste. (Do not let the soup boil.) Serve the soup garnished with the chives. Makes about 6 cups, serving 6. Gourmet December 1992