The primary ingredient in French onion soup is onions. Traditionally, onions are slowly caramelized to develop a rich, sweet flavor, which forms the soup’s foundation. The caramelized onions are then simmered with beef or vegetable broth, infused with herbs and spices. The dish is often topped with a slice of toasted bread and melted cheese, typically Gruyère, which adds to its richness. French onion soup is a classic French dish originally created as a means to use up leftover ingredients and has evolved into a beloved comfort food worldwide, known for its warm, hearty qualities.