Cook the bacon in a skillet over medium heat until crisp. Drain on paper towels and crumble into pieces. Set aside half the bacon for topping.
Add the cubed potatoes, diced onion, minced garlic, chicken broth, salt, and pepper to the crockpot. Stir in the remaining crumbled bacon.
Cover and cook on low for 6 to 7 hours or on high for 3 to 4 hours until the potatoes are very soft.
Use a potato masher to break up some of the potatoes for a thicker texture. Stir in the milk, cheddar cheese, and sour cream until the cheese melts.
Ladle the soup into bowls and top each serving with the reserved bacon and sliced green onions.