Use a 6-quart slow cooker. Add the first 4 ingredients to the insert. Cover and cook on low for 7 hours, or until the potatoes can mash easily with a fork. Mash with a large fork or potato masher, or pulse with a handheld immersion blender. Stir in the cream cheese and shredded Jack cheese. Serve when the cheese has melted completely. Garnish with green onion, if desired.