Place the cubed potatoes, diced onion, chicken broth, garlic powder, salt, and black pepper into the slow cooker. Stir to combine.
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 for a thicker texture, leaving a few chunks.
Stir in the milk, sour cream, and 1 cup of the cheddar cheese until the cheese melts.
Add most of the crumbled bacon and stir it through the soup.
Ladle into bowls and top each serving with the remaining cheese, bacon, and sliced green onions.