The short answer: Gruyère is a classic cheese for French onion soup, but it is not the only correct answer. Comté and Emmental are also traditional, excellent melting cheeses for the gratinéed top. Julia Child even used Swiss cheese with Parmesan.
So Which Cheese Goes on French Onion Soup?
If you want one easy answer, use Gruyère.
It melts well, browns beautifully and has enough nutty, savory flavor to survive sitting on top of a bowl containing what is essentially several onions that have been convinced to become dinner.
But French onion soup is not guarded by a cheese police unit.
Taste France recommends Gruyère, Comté or Emmental de Savoie for the classic gratinéed topping. All three melt well and make sense with the deeply browned onions, broth and toasted bread.
Gruyère Is Classic. Comté Is Also Very French.
Gruyère gets the most attention because it has that ideal combination of melt, salt and nuttiness.
Comté is another excellent choice and appears in plenty of French versions. Emmental works too, especially if you want something a little milder.
You can also blend them.
This is soup, not a citizenship test.
Julia Child Did Not Use Only Gruyère
This is where the “there is one sacred cheese” story gets less convenient.
In a 1963 episode of The French Chef, Julia Child demonstrated onion soup gratinée using grated Swiss cheese and Parmesan. She explicitly said you could use all Swiss, all Parmesan or a mixture.
So if somebody at dinner tells you Parmesan automatically invalidates the soup, you may politely inform them they are now arguing with Julia Child.
I recommend letting them do that alone.
What About Mozzarella?
Will mozzarella melt? Absolutely.
Will it give you the same flavor as Gruyère or Comté? No.
Mozzarella is much milder. If the goal is that deeply savory bistro-style crust on top, Gruyère, Comté or Emmental will usually get you closer.
If mozzarella is what you have, the soup will not call the authorities.
The Best Cheese Is the One That Melts and Actually Tastes Like Something
French onion soup depends on contrast: sweet browned onions, rich broth, crisp bread and a salty, browned cheese cap.
That is why Gruyère works so well. It does not merely melt. It contributes flavor.
My order of operations would be simple: Gruyère first, Comté right beside it, Emmental if that is easier to find, and a blend if the cheese drawer is feeling ambitious.
Just use enough to create the part everyone burns the roof of their mouth on because they refused to wait thirty seconds.