Ruined my homemade cole slaw salad with 2 chopped yellow onions...very acidic and bad after taste. Recipe calls for the onions. How do I not ruin the salad the next time with the onions?  
You don't say how much cole slaw you made with 2 entire chopped onions. I hope it was a lot. (I'm not sure I'm familiar with raw chopped onions at all in cole slaw.)   Did this recipe specify yellow onions? or were they perhaps supposed to be one of the "sweet" onion varieties -- Walla Walla, or Vidalia, etc. Those have a lot less bite, and are better to eat raw than the more sulfurous biting types. Maybe next time you might try using them.  Also, if you aren't wedded to this specific recipe, the obvious solution would be include less chopped onion... or even none, if you aren't happy with its contribution to the result. 