In lg pot, melt butter over medium heat. Add onions. Cook, stirring occasionally 25 mins, or until onions are golden.
Add broth, apple cider and pepper to pot. Bring to a boil. Reduce heat to low. Stir in parmesan cheese. Cook until cheese is melted.
Heat broiler. Place bread on a baking sheet. Sprinkle bread evenly w/ swiss cheese. Broil 5" from heat for 5 mins, or until cheese is melted. Ladle soup into bowls, top each bowl w/ a slice of cheesy bread.