overpowering onion

I agree on the suggestion to add lime juice or vinegar.

I've found this happening in late winter when onions have been stored for long periods of time. Or shipped from the southern hemisphere.

Very bitter...very sulfur and there's not much you can do for it except use it as a learning experience and taste the onion before using it.
I'm rather suprised this has happned to you in the summer months where onions are usually sweeter and not stored----but then again your source for onions might be ditching some long term storage nitrogen stored onions from the winter or last year and clearing out their warehouse.

Dice the onion, run under water in a fine mesh sieve. Some of the pungent flavors will dissipate. I also love using fresh sweet onions for applications like this... Guac, salas, etc. shallots also works nicely for a bit more mild flavor.

If you soak the onion slices in ice water for about 20 minutes the flavor will mellow out.

...Or some roasted or raw sweet corn.

Is there already some sort of acid in the salsa? The acid (lemon/lime juice, vinegar, etc.) will mellow out the onion the longer it marinates in the fridge.
You can also try adding a couple pinches of sugar or a squeeze of honey/agave to mellow it out. That, or some sort of other sweet ingredient (like a fruit; mango, apple, pineapple, etc.) to cut the onion flavor.
Or, just make it a 3 1/2 pound batch and leave out the onion in the last round of ingredients. I.e. more tomatoes and herbs.

You might try taking half of it out, then add more salsa ingredients (minus the onion).

Either toss the other half or try to incorporate it into some other use. Depending on what's in your salsa, you can make gazpacho, or use it in a sauce.