Place the diced potatoes, onions, butter in the bottom of the slow cooker and cover with broth.
Cover and cook on low for 6-8 hours, or high for 3-4 hours.
Once the potatoes have completely softened, use an immersion blender or potato masher to mash the potatoes. If you like a thicker soup, use a potato masher. If you like a creamier soup, use an immersion blender.
Add the milk, cream cheese, and ½ a cup of shredded cheese and continue to blend.
Add salt and pepper to taste.
Garnish with remaining cheddar cheese and green onions.