Spray 4-quart slow cooker with cooking spray. In 2-quart saucepan, melt butter over medium-high heat. Add onions, pepper and salt; cook 3 to 4 minutes, stirring occasionally, until tender. Stir in flour. Cook 1 minute, stirring constantly. Gradually stir in evaporated milk until smooth. Pour mixture into slow cooker. Stir in broth and thawed broccoli.