There was a time when I would buy two sweet onions to grill because I knew that the equivalent of one would end up between the grates. No matter how I sliced them, nor the method with which I turned, by the time the onions softened up enough and started to char I would lose the middles or even the whole slice when I flipped them. I even tried big slices cut straight through the middle of the onion, with two wooden skewers stuck through to keep everything together. In retrospect, soaking the skewers would have been a good move, but doesn't detract from my point: grilling onions can be tricky.
Therefore, I submit to you, the Lazy Kristina Method for Grilling Onions. No extra pans, no extra work. Just cut the onion into sixths (or eighths, or quarters for that matter) from pole to pole, leaving the root intact.
After tossing with olive oil, grill away. When you are ready to turn them, just pick them up by the root end. They're simply beautiful to serve just the way you are, but if you need the wedges cut up, simply slice off the root.