I've put too much onion on my macaroni salad and it overwhelms other ingredients. Is there a way to tame down the onion taste?

If it goes well with the other ingredients of your salad, you can add some mint of fresh parsley, it will balance a bit the strong onion taste.

A simple solution that works for me when I put too much of an ingredient is to scale up all the other ingredients to that same factor.

For instance, if you've used twice as much onion as you've meant to, you can double all the other ingredients for an end result with the original proportions you had in mind.

Of course, you end up with twice as much food as planned, but to me that is often better than attempting to otherwise fix the flavor and risk making matters worse.