Add butter to a large stock pot over medium heat. Sprinkle in flour once butter has melted. Stir well. Slowly whisk milk into flour, stirring frequently until thickened.
Whisk broth into flour mixture, stirring frequently. Bring mixture to a simmer. Add mashed potatoes, 3/4 cup cheese, pepper and salt.
Stir until thickened, cheese is melted, and the soup is warmed. If the soup is too thick, use an immersion blender to blend everything together or add another splash of broth or milk.
Divide soup into bowls and garnish with sour cream, remaining cheese, green onions, and bacon crumbles. Enjoy!
This nutritional information refers to each serving of the entire recipe and not just the products used as ingredients.