How do I fix a soup that turned out too salty?

The good news is you don't have to bin it. The classic fix is to dilute the soup by adding more of the non-salty components. For French onion soup, that means adding more broth - either beef or vegetable broth depending on what you used. Start by adding maybe a cup at a time and taste as you go. It's easier to add more liquid than to remove salt, so go slow. Stir it well between additions so the salt distributes evenly and you get an accurate taste.

If you've already added a decent amount of broth and it's still pretty salty, there's another trick I've used when I messed up dinner before heading to pick up passengers: add diced potatoes or a slice of bread to the pot. Both absorb salt pretty well. Cook the potatoes until they're soft enough to break apart easily, then fish them out (or leave them in if you don't mind extra texture). The bread basically dissolves as it absorbs the salt, so that's a cleaner approach if you're worried about ruining the soup's consistency.

One thing though - French onion soup relies on that caramelized onion flavor being concentrated, so be prepared that the final result might taste a bit more watered down than you intended, even after the fix works. But it'll be edible and honestly most people won't notice the difference if the onion flavor is still there. Just make sure you don't go overboard with salt next time; a pinch at a time goes a long way in something like this.