Just chop the onion into fairly small pieces and freeze it raw. I do that all the time. I hate chopping onions, and with only hubby and me, I don't always use a whole onion at once. It's easy to cook the frozen pieces, or just dump them in the crockpot with the other ingredients.
...I actually wear swimming goggles when I chop onions, they make my eyes burn and water so badly I can't see otherwise!