Cook the bacon in a skillet over medium heat until crisp. Remove the bacon, drain it on paper towels, and crumble it into pieces. Set aside half the bacon for topping later.
Add the diced potatoes, onion, garlic, and chicken broth to the slow cooker. Stir in the crumbled bacon pieces from the first half.
Cover and cook on low for 6 to 7 hours until the potatoes are very soft.
Use a potato masher to break up some of the potatoes right in the slow cooker for a thicker texture. Stir in the milk, cheddar cheese, and sour cream until the cheese melts and the soup turns smooth.
Taste and season with salt and black pepper as needed. Ladle into bowls and top each serving with the reserved bacon and sliced green onions.