In a large pot, brown beef onion, celery, and carrots until beef is browned.
Stir in Italian seasoning. Add broth and potatoes. Bring to a boil, then simmer until potatoes are tender, about 10-12 minutes.
In a separate saucepan, melt the remainder of butter and stir in flour. Add the milk, stirring until smooth.
Gradually add milk mixture to the soup, stirring constantly. Bring to a boil and reduce heat to simmer. Stir in cheese. When cheese is melted, *add sour cream and heat thoroughly but do not boil*.