The primary ingredient used in traditional French onion soup is onions. Typically, yellow onions are caramelized to develop their natural sweetness, which is the foundation of the soup's deep flavor. Along with onions, the soup is prepared with beef or vegetable broth and is often seasoned with thyme and a splash of white wine. Once cooked, the soup is topped with crusty bread and melted cheese, usually Gruyère, and then broiled until bubbly and golden. French onion soup is celebrated for its comforting qualities and rich, savory profile, making it a classic dish in French cuisine.