Place the diced potatoes, chopped onion, and minced garlic into the slow cooker.
Pour in the chicken broth and stir to combine.
Add salt, black pepper, and dried thyme to the slow cooker and stir gently.
Cover and cook on low for 6 hours, or until the potatoes are tender when pierced with a fork.
In a small bowl, whisk together the whole milk and all-purpose flour until smooth.
Once the potatoes are tender, use a potato masher to mash some of the potatoes in the slow cooker, leaving some chunks for texture.
Stir the milk and flour mixture into the slow cooker.
Add the unsalted butter and shredded cheddar cheese, stirring until the cheese is melted and the soup is creamy.
Stir in the sour cream until fully incorporated.
Taste and adjust seasoning with additional salt and pepper if needed.
Serve the soup hot, garnished with crumbled bacon and sliced green onions.