Which ingredient is used as a topping in the French Onion Soup?

In French Onion Soup, the traditional topping includes melted provolone cheese. This cheese is known for its excellent melting properties, which create a rich and gooey layer over the soup when broiled. Provolone complements the caramelized onions and beef broth base of the soup, adding depth and a delightful texture.

The other cheeses listed, such as melted cheddar, Parmesan, and feta, do not have the same classic application in this particular dish. Cheddar may be used in some variations but is not traditional to the classic French Onion Soup. Parmesan tends to be used more as a garnish rather than a main topping due to its strong flavor, while feta would provide a tangy flavor that is not characteristic of this soup's profile. Using provolone ensures that the dish remains true to its roots while providing the desired flavor and texture experience.