In skillet, cook bacon over medium heat, until browned & crispy. Remove from skillet, and drain on paper towels, then refrigerate until dinner time.
In bacon drippings, cook onion over medium heat 4-5 minutes, stirring frequently, until tender.
Spray slow cooker with cooking spray. In cooker, mix onion, potatoes, celery, broth, salt & pepper. Cook in slow cooker on low for 6-7 hours.
In small bowl, beat flour and half and half (or milk) with wire whisk until well blended; stir into soup.
Increase heat on slow cooker until high. Cover; cook about 30 minutes longer or until thickened.
Stir in cheese until melted & smooth. (If you want fewer lumps in your soup, you’ll want to mash the potatoes a bit with a potato masher.)
Crumble bacon, serve on top of soup.