The main ingredient in a traditional French onion soup is onions, which are caramelized to develop a deep, rich flavor. The soup typically features a base of beef broth, although some variations use chicken or vegetable broth. The caramelization process enhances the sweetness of the onions, balancing beautifully with the savory broth. Often, the soup is topped with a crust of melted cheese, traditionally Gruyère, and served with crusty bread, creating a comforting and hearty dish. French onion soup is a classic representation of French cuisine, known for its rustic yet sophisticated appeal.