Serves 6 people
1 large Spanish onion, thinly sliced and separated into rings
¼ cup, plus 2 tablespoons extra-virgin olive oil
Kosher salt and freshly ground black pepper
6 tablespoons (¾ stick) butter, softened
¼ cup, plus 2 tablespoons packed light brown sugar
Preheat a gas grill or prepare a fire in a charcoal grill.
Cut six 14- by 12-inch sheets of heavy-duty foil and lay them out on a work surface, with a short end toward you.
Layer one-sixth of the potato slices and onion rings on the bottom half of each sheet of foil, leaving an inch or so of space all around. Brush the tops and sides of the vegetables with olive oil, using 1 tablespoon for each packet, and sprinkle with salt and pepper.
Dot the top of each pile of potatoes with 1 tablespoon of the butter and sprinkle with 1 tablespoon of the brown sugar.
Fold about ¼ inch of the sides and bottoms of the foil to seal the edges, then fold over again to make a tight seal.
Place the packets on the hottest part of the grill and cook for 15 minutes. Transfer to a large platter or baking sheet and let stand for 10 minutes.
Serve in the foil, warning your guests about the hot, fragrant steam that will arise when they cut into the packets. Be sure to spoon some of the syrupy juices over the sweet potatoes.