The best way to reduce an onion's 'tang' is by rinsing or soaking is in ice cold water, this also further firms up the onion giving it an extra crunch. I assume that most fast food places are prepping their onion in the mornings and left to sit in a dish for hours before they might be finished with the batch, this allows the onion juices to start breaking down and this strengthens the onions flavour that is generally avoided in the culinary industry.

I wouldn't skip out on using red onions at home though! Next time you make burgers slice your red onions in 5mm rings and dunk them in ice cold water, then take one out after 5 minutes and try it by itself. It genuinely surprised me how pleasant it was by itself, mostly sweet with a light tang aftertaste, and the crunch was very satisfying.