Spray 5-quart slow cooker with cooking spray. Heat 12-inch skillet over medium heat; cook bacon 10 to 15 minutes or until crispy. Transfer to paper towels to drain. Crumble 3 slices of the bacon; cover and refrigerate until ready to serve. Crumble remaining 3 slices, and transfer to slow cooker.
Pour all but 1 tablespoon bacon drippings out of skillet. Add onion to skillet; cook 4 to 6 minutes, stirring occasionally, until tender. Transfer onion mixture to slow cooker. Stir in broth, potatoes and ranch dressing mix.
Cover; cook on Low heat setting 5 to 6 hours or until bubbly and potatoes are tender.
Increase heat setting to High; stir in whipping cream. In small bowl, beat cornstarch and water with whisk. Beat cornstarch mixture into soup; cover and cook about 15 minutes or until slightly thickened.
Add Kraft™ Velveeta™ cheese to slow cooker; stir until melted. Add 1 ½ cups of the Cheddar cheese; stir until melted. Serve soup with remaining ½ cup Cheddar cheese and reserved crumbled bacon.
