I'll lower the heat and, I think, watch the burgers carefully, hoping to take them off the grill at the exact moment the burgers are done, but before the onions can burn.
Now here are a couple of suggestions for making your mini-burgers even better:
1. For slicing the onions, order, from Williams-Sonoma, a Kyocera ceramic slicer -- the yellow one (the green one is for juliennes): http://ww1.williams-sonoma.com/cat/pip.cfm?pkey=xsrd0m1%7C15%7C0%7C%7C%7C%7C%7C%7Cceramic&gids=cw150&cmsrc=sch
They're fantastic. Within seconds, you can turn one onion into a mountain of paper-thin onions. By the way, I use sweet onions, such as Vadalia or its equivalent.
2. For buns, you want Martin's Potato Rolls: http://www.mfps.com/products.html
The ones I am talking about are on the upper-right on the Web page. They taste much better than ordinary buns and, as you can perhaps see in the photo, they are available in a 12-pack of mini-buns -- the perfect size and shape for mini-burgers. Hopefully, they are in your local market; otherwise, you probably can contact them via the Web site and ask them for a location in your area.
And, if you like hot dogs, too, then try Martin's Long Potato Rolls (second row, middle photo). Once you do, you'll never be satisfied with ordinary hot dog buns again.