In Dutch oven or soup kettle, bring water to boil. Add broccoli, carrots, and celery; boil 2 to 3 minutes. Drain; set vegetables aside. In the same kettle, saute onion in butter until tender. Stir in flour to form a smooth paste. Gradually add the broth and milk, stirring constantly. Bring to boil; boil and stir 1 minute. Add vegetables. Stir in parsley, salt and thyme. Reduce heat; cover and simmer for 30 to 40 minutes until vegetables are tender and flavors are combined. Ladle soup into bowls and garnish with croutons.