If you want something really good for he BBQ, cut the top (not root) end off of the onion so there's a flat surface. Leave skin intact. Place on a double square of foil with flat side up. Place large dollop of butter on top. Pull foil up and seal tightly. Place on hot BBQ with sealed top up so butter doesn't run out and flare up. "Bake" until everything else is done. Oh 15-20 minutes depending upon size of onion.
Unwrap, pop off the skin, cut up and serve with the rest of your meal. The onion is sweet and tasty.