Since the salad is already assembled, I think the best suggestion was mentioned in the first reply - increase the total volume of the salad by adding more of the other ingredients.

How thickly is the onion sliced? This can make a big difference in terms of how much presence the onion has.

Always worth tasting the onion first and if it's a strong one, soak it for a bit in cold water, then rince.

Not sure this is fixable, since it sounds like your pasta salad is already composed. If you can pick the onion out, do that. I'd add some chopped parsley, and some more chopped veg, too, along the lines of campagnes idea to add bulk.

Otherwise, serve Tic Tacs for dessert. :)

oil and vinegar. if you have an end product try a drizzle of evoo -- the mayo should work itself if it has time to sit.

If you are using vinegar in your dressing, take a few minutes to soak your chopped onion in it before using it in the dressing. It takes that sharp bite right out of the onion and it seems to sweeten them almost. I got this idea from a pickled onion recipe I tried once. I've used it ever since in salads.

Oh yes. Slice the onions pole to pole. This way the onions' cell walls do not rupture and the will be less pungent. Slices against the grain will rupture more cell walls and cause more of the chemical to be released that makes you cry as well as taste more onion-y. Also, if an even more tame onion is desired giving them a ten minute soak in ice water will do the trick.

Thinly slice or chop the onion, put into a strainer, and run cool water over it for about 1/2 a minute, tossing or running your fingers through them, to rinse off some of the pungent component that makes the onion taste strong raw. I believe that this also helps prevent them from oxidizing so quickly, but I'm not 100% on that.

Is it at all possible to fish out some of the onion? and maybe to add a bit more pasta and increase the mayo and other seasonings? basically to increase the volume of the other ingredients? I'm kinda iffy about raw onion in salads so I don't have much insight to offer.. hope it works out in the end.