Crockpot Cheesy Potato Soup

6 Bacon slices
1 1/2 Cups onion, chopped
5 Cups russet potatoes, peeled & diced
1 Medium stalk celery, chopped
1 (32 oz) Carton chicken broth
1/2 Teaspoon salt
1/4 Teaspoon pepper
1/2 Cup all-purpose flour
1 1/2 Cups half & half
1 Cup cheddar cheese, shredded
1 Cup Velveeta cheese, cubed

In 12" 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 a 4 quart crockpot with cooking spray. In cooker, mix onion, potatoes, celery, broth, salt and pepper; cover and cook on LOW for 6 to 7 hours.
In small bowl, beat flour and half & half with wire whisk until well blended; stir into soup. Increase heat setting to HIGH. Cover and cook about 30 minutes longer or until thickened. Stir in cheese until well melted. Crumble bacon; sprinkle over soup.