In 12-inch skillet, cook bacon over medium heat, turning occasionally, until browned and crispy. Remove from skillet, reserving fat in skillet. Drain bacon on paper towel, then refrigerate. In same skillet, cook onion in bacon fat over medium heat 4 to 5 minutes, stirring frequently, until tender.
Spray 3 1/2- to 4-quart slow cooker with cooking spray. In cooker, mix onion, potatoes, celery, broth, salt and pepper.
Cover; cook on Low heat setting 6 to 7 hours.
In small bowl, beat flour and half-and-half with wire whisk until well blended; stir into soup. Increase heat setting to High. Cover; cook about 30 minutes longer or until thickened. Stir in cheese until well melted. Crumble bacon; sprinkle over soup.