Spray inside of 3- to 4-quart slow cooker with cooking spray. In large bowl, mix all ingredients except onions. Stir in half of the onions; spoon mixture into slow cooker.
Cover; cook on Low heat setting 3 to 4 hours, stirring after 1 to 1 1/2 hours.
About 5 minutes before serving, in 6-inch skillet, heat remaining half of onions over medium-high heat 2 to 3 minutes, stirring frequently, until hot. Stir bean mixture; sprinkle with onions.