Choose firm, medium onions — yellow, white, or sweet (Vidalia) all work. Peel, then trim root and stem ends flat. For slices, cut crosswise ½ inch thick — thinner falls through grates, thicker won't soften. Brush both sides with oil, season with salt and pepper just before grilling. For foil-wrapped whole onions, core out the center, pack with butter and seasonings, then wrap tightly in a double layer of foil to hold the juices.

Sliced rounds: Preheat grill to medium heat (177°C – 232°C). Oil grates. Lay oiled slices in a single layer over direct heat. Close lid and cook ~8 minutes total, flipping once at 4 minutes per side. Look for clear grill marks on both sides and translucent, softened edges. Don't move slices before flipping — you'll lose the marks and caramelized crust. Foil-wrapped whole onions: Seal packets tightly and place over indirect heat. Cook 30–60 minutes depending on size and desired tenderness. Smaller or firmer: 30–40 min. Larger or fully caramelized: up to 60 min or beyond. Don't puncture foil — tears cause flare-ups and dry out the onion. Keep lid closed to hold consistent heat.

Sliced onions are done when both sides have grill marks and layers are soft and slightly translucent — they should yield to tongs without falling apart. For foil-wrapped whole onions, open the packet away from your face, then test with a fork or skewer; it should slide through the center with little resistance. Remove tops, cut into bite-sized chunks, and transfer to a dish with all the foil juices — don't discard them. Avoid cutting sliced onions too thin or they'll fall through grates and overcook. Seal foil packets tightly or steam escapes and onions can burn. Serve immediately as a side, burger topping, or with grilled meats.