Add potatoes, broth, carrot, onion, parsley, celery seed, salt and pepper. Cover and simmer until potatoes are tender, about 15 minutes.
Combine flour and milk together until smooth; add to soup.
Bring to a boil; boil and stir for 2 minutes.
Add cheese and stir until cheese is melted and the soup is heated through.
Garnish with green onions.