8 medium leeks (3 pound), trimmed, leaving white and pale green parts only, and chopped
1 medium onion, chopped
1 large carrot, chopped
2 celery ribs, chopped
1 teaspoon salt
1/2 teaspoon black pepper
1 stick (1/2 cup) unsalted butter
1 small boiling potato (6 ounces)
1/2 cup dry white wine
3 cups chicken stock or reduced-sodium chicken broth (24 fluid ounces)
3 cups water
1 Turkish bay leaf or 1/2 California
1 1/2 cups fresh flat-leaf parsley leaves
1/4 cup all-purpose flour
1/2 cup chilled heavy cream