1Spray 4-quart slow cooker with cooking spray. In 12-inch skillet, melt butter over medium-high heat. Cook onion in butter 4 minutes, stirring occasionally, until tender. Stir in flour. Cook 1 minute, stirring constantly. Gradually stir in milk until smooth. Pour mixture into slow cooker. Stir in broth, broccoli, pepper and salt.
2Cover; cook on Low heat setting 4 hours or until bubbly.
3Add cheese cubes to slow cooker; stir until melted. Add 1 1/2 cups Cheddar cheese and the Parmesan cheese; stir until melted. Sprinkle individual servings with additional Cheddar cheese.