The primary ingredient in a traditional French onion soup is, unsurprisingly, onions. This dish highlights caramelized onions, which develop a rich, sweet flavor when slowly cooked down. Typically, the onions are sautéed with butter until they're golden brown, creating a deep, aromatic base for the soup. The dish is often combined with beef broth, which enriches the flavor profile. It's traditionally topped with a layer of toasted bread and melted cheese, usually Gruyère, making it a comforting and flavorful staple of French cuisine.