In a large pot, melt the butter over medium heat and whisk in the flour to form a roux.
Gradually add the milk, stirring constantly, until thickened.
Add the chicken broth, potatoes, salt, and pepper. Simmer for 20 minutes or until the potatoes are tender.
Use an immersion blender to blend the soup to your desired consistency.
Stir in the cheese and sour cream until melted and well combined.
Serve hot, garnished with green onions and crumbled bacon.