Spray 3 1/2-quart slow cooker with cooking spray. In 1-quart saucepan, melt butter over medium-high heat. Cook onion in butter 3 minutes, stirring occasionally, until tender. Stir in flour; cook and stir 1 minute. Reduce heat to medium. Gradually stir in half-and-half; cook 3 to 5 minutes, stirring constantly, until thickened. Pour into slow cooker. Stir in broth and broccoli.