Combine potatoes, chicken stock, onion, salt, and pepper in a slow cooker. Cook on High until potatoes are tender, about 4 hours.
Meanwhile, place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain bacon on paper towels and wipe out most of the grease from the skillet.
Puree potatoes in the slow cooker with an immersion blender until smooth. Add 1/2 cup Cheddar cheese and 1/2 of the cooked bacon, set temperature to Low, and cook for 1 to 2 hours.
Melt butter in the skillet over medium-low heat. Whisk in flour and stir until the mixture becomes paste-like and light golden brown, about 5 minutes. Add heavy cream and sour cream and whisk until thickened. Pour into the slow cooker and cook on Low for 1 more hour.
Ladle into bowls and garnish with remaining bacon, remaining Cheddar, sour cream, and scallions.
If you don't have an immersion blender, you can use a potato masher instead.