Spaghetti sauce bubbling on the stove reduces in volume the longer it cooks. The concentrated liquid intensifies the flavor of the ingredients, but it can also result in overly salty sauce. You don't have to start over or double the ingredients, making a bigger batch than you really need. Depending on the severity of the salt, you can use one or all of three techniques to bring balance back to the sauce.
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Stir the sauce as you add a pinch at a time of brown sugar. Taste the sauce after each addition so that you don't make the sauce noticeably sweeter. If the salt still overpowers the sauce but you don't want to add more sugar, move on to the next step.
Add a squirt of fresh lemon juice to the sauce and stir well. The acidity in lemon juice cancels out the flavor of excess salt without changing the taste of the spaghetti sauce. If you still detect a hint of saltiness, move on to the next step.
Cut a large potato into quarters. Place the potato chunks in the spaghetti sauce and simmer it for 20 minutes. Remove the potato pieces and discard them. The starchy potatoes absorb the excess salt.