Cook bacon in a skillet until crisp. Move bacon to a plate. Leave a spoon of drippings in the pan.
Add onion and garlic to the skillet. Stir until soft. Add butter and let it melt. Sprinkle in flour and whisk to form a blond paste.
Whisk in chicken broth. Stir until smooth. Pour this into the slow cooker.
Add potatoes, water, salt, and pepper. Cover and cook on high for 3 to 4 hours or on low for 6 to 7 hours until potatoes are tender.
Stir in heavy cream and most of the cheese. Mash a few potatoes against the side to thicken.
Ladle into bowls. Top with bacon, green onions, and more cheese.