Over-salting: It could happen in a million different ways. Maybe, like in those old Three Stooges films, you keep your box of salt on a shelf directly above the stove and a cat jumps up there and dumps the whole thing into your soup. Maybe the recipe you're following called for Kosher salt, and you used table salt (which is twice as salty by volume) instead. Regardless of how it happened, the question is, can you fix a soup or sauce that becomes over-salted?
The Potato Trick
We've all heard about the magical "just add a potato" solution to fixing an over-salted soup or sauce. The theory is that if you add a potato to a salty soup and simmer it, the potato comes out salty. If there's salt in the potato, it stands to reason that you've removed some of the salt from the soup.
Is this piece of culinary folklore really true? Or is it like the idea that holding a slice of bread in your mouth when you chop onions will stop your eyes from watering?
Well, potatoes don't pull salt out of anything. They do absorb water, though—and if that water happens to be salty, they'll absorb salty water. But they're not absorbing salt in particular. Potatoes are amazing, but they're not capable of reverse osmosis. It's more like using a sponge to soak up a spill.
So in theory, if you added enough potatoes to absorb all the water in your super-salty sauce, then removed the potatoes and added more water, you'd end up with a sauce that wasn't too salty.
Dilute It or Drain It
You could've accomplished the same thing by skipping the potatoes altogether and simply adding more water. That's because there's no way to remove salt from something. All you can do is dilute it.
Thus the best way to fix an over-salted sauce or soup is to make a bigger batch of whatever it is. Tomato sauce too salty? Add more crushed tomatoes. Soup too salty? Add more water. Yes, you'll likely have to add more of the other ingredients as well otherwise the soup will be too watery, but don't try to reduce it by simmering. You'll just evaporate the water you added and end up re-concentrating the saltiness.
Another option, if you don't have enough of the other ingredients to increase the recipe, is to just pour out a bunch of the liquid and then add more. Depending on what stage of the cooking you're at, that might be easier.
In some cases, when all is said and done, you may have to face the painful reality that your soup, sauce, or stew can't be salvaged. Mistakes cost money, and cooking mistakes are no exception. But if you've learned from it, it's not a total loss. If nothing else, you'll have a great story about the cat and the box of salt.
You'll also be able to save your potatoes for something more enjoyable.