Roast 30 minutes- gas grill, middle shelf; charcoal, indirect heat.
Remove from grill onto disposable surface (there will be lots of carbon), cut the bottom off the onion and the soft, steamed interior should come out easily, leaving behind the charred skin layers.
Remove to clean plate.
Slice/ chop the onions and toss with seasonings.
(We serve them over buttered baked potatoes and top with grated cheddar cheese, fresh ground pepper and Tabasco. We stick a clean, food-use-only non-anodized aluminum nail or tent stake through the stem / tail ends of the onion before it goes on the grill. We find that it helps with even heat distribution - it works great for potatoes too.)