At a BBQ not too long ago, a friend prepared some onions for the grill. They were so tasty, I just had to share the how-to.
This works best with Walla Walla sweet onions. They are just so mild & yummy, & not as strong or onion-y as a regular onion. If that makes sense.
Hopefully this picture helps you see the details well enough. Simply cut a "cone-ish" shape out of the top of the onion. Sprinkle the inside with your favorite spices. (Seasoning salt or garlic or whatever you fancy.) Plop a couple tablespoons of butter inside also. Then smoosh the top back on, set them on the grill & cook until tender---about 45 minutes.