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

The classic fix is dilution, and with French onion soup you've got some good options. Your best bet is to add more broth - beef broth ideally to keep it authentic. Start by adding maybe a cup at a time and taste as you go. This is tedious but beats dumping the whole pot. If you don't have broth on hand, water works too, though you'll lose some depth. The downside is you'll end up with more soup than you planned, but honestly that's not the worst problem to have.

If dilution alone doesn't cut it, you can try the potato trick. Peel and quarter a couple of potatoes and simmer them in the soup for about 15-20 minutes. Potatoes absorb salt pretty well. Fish them out after and discard them. I've had mixed results with this method - sometimes it works great, sometimes it only takes the edge off. Worth trying before you give up though.

One thing that might actually help with French onion specifically is balancing the salt with other flavors. A splash of good vinegar or a bit of sugar can sometimes make oversalted soup taste more rounded and less aggressively salty. Doesn't fix it entirely but makes it more bearable. That said, if it's genuinely inedible after trying these, starting over might honestly be faster than wrestling with it. Just go easier on the salt next time - you can always add more but you can't take it out.