Mix remaining ingredients in a bowl, add potatoes and toss to coat.
Place potato mixture onto ½ of the foil; fold over other half and seal edges by pinching halves together to form a packet. Slit the top of the foil packet several times to vent.
Place the packet on the grill when the coal is at its hottest and grill for 30 minutes. At about the halfway mark, you may want to open the foil and stir the potatoes, but be careful not to let the oil run out (i.e. don’t simply turn packet over) or you will cause fire, soot and general mayhem. Not good. But the potatoes are and you don’t want to have to share them with the fire department, so be careful.