Sauté first five ingredients with ¼ cup butter for 10 minutes. Add broth, potatoes and ground beef. Bring to a boil. Reduce heat, cover and simmer for 10-12 minutes or until potatoes are done. Meanwhile, in a small pan add the remaining butter, add flour and cook and stir until bubbly 3-5 mins. Add to soup mixture and bring to a boil for 2 minutes. Reduce heat to low, add cheese, milk, salt and pepper. Cook and stir until cheese is melted. Add sour cream when ready to eat.