In 3- to 4-quart slow cooker, mix potatoes, onion, celery, broth and water.
Cover; cook on Low heat setting 6 to 8 hours.
In small bowl, mix flour into milk; stir into potato mixture. Increase heat setting to High. Cover; cook 20 to 30 minutes or until mixture thickens. Stir in cheese until melted. Garnish individual servings with bacon and green onions. Sprinkle with pepper if desired.