In a large skillet, saute onions in butter until golden brown, for 8 minutes. In a medium saucepan, combine beef broth, salt, pepper and onion: bring to boiling. Reduce heat and simmer for 30 min. In broiler, toast French bread. Sprinkle each slice with grated cheeses. Broil just until cheese is bubbly, about 1 min. To serve, pour soup into individual soup bowls. Float toast on top, cheese side up.