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 ½-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.