Mellowing out red onion in a bean/grain salad?
Don't know if this helps now - I'm not sure you can do anything after the fact - but I've read that soaking raw onions in ice water helps to take some of the sting out if they are really powerful. I do this when they're raw in a salad like this, or just use less than recipe calls for!
unfortunately, there's not much you can do except to add more of everything else to distribute the onion flavor over a larger volume of ingredients. next time, soak the raw onions in lemon or lime juice (whichever flavor complements your dish better) before adding. i've found that the citrus is even more effective than ice water for mellowing out sharp raw onions.
I just had a similar predicament with a carrot salad with raw red onion; the onion flavor stared overwhelming everything else after it sat overnight in the fridge. i added a little sugar and some medium diced cucumber, which mellowed it enough for it to taste good. The onion is still rather strong though, maybe I'll try lemon juice next.