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
Wash sliced leeks in a large bowl of cold water, agitating them, then lift out and drain well in a colander.
Cook leeks, onion, carrot, celery, salt, and pepper in 4 tablespoons butter in a 5- to 6-quart heavy pot over moderate heat, stirring occasionally, until softened, about 8 minutes. Peel potato and cut into 1/2-inch cubes, then add to onion mixture along with wine, stock, water, and bay leaf. Bring to a boil, then reduce heat and simmer, partially covered, until vegetables are tender, about 15 minutes.
Stir in parsley and simmer soup, uncovered, 5 minutes. Discard bay leaf and keep soup at a bare simmer.
Melt remaining 4 tablespoons butter in a 1-quart heavy saucepan over moderate heat, then add flour and cook roux, whisking, until golden, about 3 minutes. Remove from heat and add 2 cups simmering stock (from soup), whisking vigorously (mixture will be thick), then whisk mixture into remaining soup and return to a simmer, whisking.
Blend soup in 4 batches in a blender until smooth (use caution when blending hot liquids), about 1 minute per batch, transferring to a 3- to 4-quart saucepan. Reheat if necessary, then season with salt and pepper.
Beat cream in a bowl with an electric mixer at medium speed until it almost forms soft peaks. Serve soup topped with cream.
Soup is best when made 1 to 3 days ahead (to allow flavors to develop); do not whip cream ahead. Chill soup, uncovered, until completely cooled, then cover. Reheat, thinning with water if necessary.