In a small skillet, heat butter over medium-high heat. Add onion; cook and stir 3-4 minutes or until tender. Transfer to a 3- or 4-qt. slow cooker. Add broccoli, broth, milk and pepper., Cook, covered, on low 3-4 hours or until broccoli is tender. Stir in process cheese until melted. Add shredded cheeses; stir until melted. Just before serving, stir soup to combine. Top servings with additional cheddar cheese.