Spray 3 1/2- to 4-quart slow cooker with cooking spray. Reserve 1/4 cup broth in small bowl; cover and refrigerate. In cooker, mix onions, potatoes and remaining broth.
Cover; cook on Low heat setting 8 hours.
Increase heat setting to High. In small bowl, beat cornstarch and reserved broth until smooth. Add to cooker; stir. Cover; heat to simmering, and cook 10 to 15 minutes longer or until thickened.
Stir in cheese until well melted. Sprinkle servings of soup with green onions.