The main ingredient in French onion soup is onions, which are slowly caramelized to bring out their rich sweetness before being simmered in a flavorful beef or vegetable broth. The dish often includes ingredients like butter, garlic, and thyme, which enhance its flavor. Traditional French onion soup is typically topped with a slice of crusty bread and a generous layer of melted cheese, which is then broiled until golden and bubbly. This comforting soup is a classic French recipe that showcases the simplicity and depth of flavors achievable with just a few essential ingredients.