We had never seen this particular method before, so we did a little searching.
It seems that rubbing a hot grill with half a cut onion (cut side down) will loosen up baked on grit and grime and basically take it all right off the grill. It's best to heat the grill super hot first and burn down any remaining food or crud, then rub it hard with an onion stuck on the end of a fork (as seen above).
We haven't tried this yet, but we love that it's a totally green option too! No icky grill cleaning chemicals.
Have you ever cleaned your grill like this?