Heat olive oil in a large saucepan over medium heat. Add onion and celery, and sauté 5 minutes or until onion is soft. Stir in potato and next 5 ingredients (potato through bay leaf). Bring to a boil; cover, reduce heat, and simmer 12 minutes or until potato is tender.
Combine milk and flour in a small bowl, stirring with a whisk until smooth. Add flour mixture and clams to saucepan, and bring to a simmer. Cook 2 minutes or until mixture begins to thicken, stirring frequently. Discard bay leaf. Ladle soup into bowls; garnish with chopped parsley, if desired.