Tear off two large pieces of heavy duty aluminum foil.
Cut the potatoes into ¼-inch slices and add to the foil. Sprinkle with BBQ rub.
Top with onions and dot with cubes of butter.
Seal the packets and put them on the warming tray of the grill. Cook for about 20 minutes. Pierce the packets with a sharp fork to check if the potatoes are soft and cooked through.
Recipe by the south in my mouth at http://www.thesouthinmymouth.com/2013/09/15/grilled-potato-packets/