Place Onion and butter in a 1 1/2 qt slow cooker coated with cooking spray.
cover and cook on low for 6 hours or until onion is tender.
Stir in broth and pepper.
Cover and cook 2 hours longer or until heated through.
Ladle soup into ovenproof bowls. Top each with a slice of toast and cheese. Broil 4-6 inches from the heat for 2-3 min or until cheese is melted. Serve