Add the potatoes, onion, chicken broth, bacon, salt and pepper to the slow cooker. Set it on low for 6 hours or high for 4 hours. 

In the last 10 minutes of the slow cooker’s cooking time, melt the butter in a saucepan on the stovetop at medium heat. Add the flour and whisk for 2 minutes to cook off the floury taste. 

Add the heavy cream to the pan and stir almost continuously until the mixture has thickened and would coat a spoon. Add milk or water to thin if needed. Add the sour cream and stir to combine, remove from heat. 

Add the roux from the saucepan into the slow cooker and stir to combine. 

Using an immersion blender or working in batches in an upright blender, blend the soup till partly smooth. You can make it totally smooth if you prefer.

Taste the soup and adjust the seasonings as needed. Top with shredded cheese, bacon, and scallions when serving. Enjoy!