COOK and stir bacon in large skillet on medium heat until crisp. Remove bacon from skillet with slotted spoon, reserving 2 Tbsp. drippings in skillet. Drain bacon on paper towels; refrigerate until ready to use. Add onions to drippings in skillet; cook and stir 5 min. or until crisp-tender. Stir in flour; cook and stir 1 min. Add 1 cup broth; cook and stir 2 min. or until sauce comes to boil and thickens. Pour into slow cooker. Stir in remaining broth and potatoes; cover with lid. Cook on LOW 8 to 10 hours (or HIGH 5 to 6 hours).
USE slotted spoon to transfer 4 cups potatoes to medium bowl; mash until smooth. Add 1-1/2 cups cheese to remaining mixture in slow cooker; stir until melted. Stir in mashed potatoes and milk; cook, covered, 5 min. or until heated through.
PLACE bacon on paper towel-covered microwaveable plate. Microwave on HIGH 20 to 30 sec. or until heated through. Serve soup topped with bacon, remaining cheese, sour cream and chives.