1. In a large bowl, toss sweet potato slices and onion wedges with the olive oil, thyme, ketchup, chili powder, salt, and pepper.
2. Set out six 12- by 14-inch sheets of heavy-duty aluminum foil (see notes) on the counter, shiny side down. Distribute the potato mixture evenly among the sheets, spooning into the center of each sheet. Fold the top edge of each foil sheet down over the potato mixture. Crimp all the edges together to create a tight seal.
3. Prepare your grill for indirect heat (see "How to set up your grill for indirect heat" below). The temperature inside your grill should be between 350° and 400° (insert a long-stemmed thermometer through lid vent to measure temperature). Place packages on the grill, overlapping slightly if needed, and cover barbecue. Cook until potatoes are tender when pierced, 25 to 30 minutes. If cooking with the Spice-Rubbed Smoke-Roasted Chicken, add to grill during the last 30 minutes of cooking (see notes).
4. Remove packages from grill. Let cool 10 minutes, then serve each packet with a wedge of lime to squirt over the vegetables.
How to set up your grill for indirect heat.
On a charcoal barbecue: Ignite about 60 briquets on the firegrate of your barbecue. Let burn until coals are dotted with ash, about 25 minutes, then push coals to one side and lay a drip pan on the other side. Set grill in place.
On a gas barbecue: Turn heat to high and close lid for at least 10 minutes. Adjust heat to medium and turn off burners as needed to create a hot and a cool zone for indirect cooking. Set drip pan in cool zone; set grill in place.