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