To a 6 or 7-quart slow cooker, add potatoes, onion, chicken broth, evaporated milk and season with salt and pepper to taste. Cover with lid and cook on HIGH heat for 4 hours or LOW heat for 8 hours. To check for doneness, poke a piece of potato with a knife. It if easily slides through, it is cooked.
Ladle out 2 cups liquid from soup mixture in crock pot into a liquid measuring cup, set aside. In a medium saucepan, melt butter over medium heat. Add flour and cook, stirring constantly, 2 minutes. While whisking, slowly pour 2 cups liquid in measuring cup into butter mixture. Pour butter mixture into slow cooker and stir to blend.
If desired, mash potatoes with a potato masher to break down into smaller pieces or use an immersion blender to puree. (High recommended. Be careful when using an immersion blender. If it splatters on you it can be quite hot).
Cover and cook on HIGH heat until thickened, about 10 minutes. Turn heat off (or to warm), stir in sour cream. Serve warm topped with cheddar, bacon and green onions.