In a soup kettle, combine first nine ingredients & bring to a boil.
Reduce heat, cover, and simmer for 15 - 20 min(until potatoes are tender).
As soup is simmering, take a large skillet and cook ground beef and mushrooms in butter over medium heat. Drain when done.
Add beef and mushrooms to soup.
Stir in 4 C. of milk and heat through.
In a small bowl, combine flour and 1C. milk. Whisk until smooth and gradually stir into soup.
Bring soup to a boil & stir for 2 min.
Reduce heat; stir in cubes of Velveeta until melted.