Technique #3: Wrap potatoes in foil to keep them moist and to blend flavors
One of the easiest and most delicious ways to cook potatoes on the grill is to toss them with a lot of the same goodies you might use for roasted potatoes and
wrap them tightly in foil. Put the package on the grate directly over the heat, cover the grill, and in 40 minutes you have a delicious side dish.
Grilled this way, the potatoes steam to a moist but firm texture and bathe in the juices of roasting aromatics. Garlic, onions, and peppers practically melt into the potatoes for extra flavor. And if your coals are hot enough, the potatoes closest to the outside of the package will have nicely charred edges, too. The package can sit off the grill, unopened, for up to 15 minutes, while you grill the main course.
A few things to keep in mind: First, before wrapping the potatoes in the foil, spread them out so that the pile is an even thickness, about 1-1/2 inches deep. This will let more potatoes come in close contact with the heat of the fire, ensuring even cooking. Second, mound the coals so that they're 5 to 6 inches below the center of the grate (but no closer). Then set the package directly over the hottest spot. Ideally, the potatoes on the bottom will be just beginning to char when they're flipped after 20 minutes. Last, be sure to cover the grill completely (leave the top and bottom vents open) so the potatoes bake in the oven-like atmosphere. You can cook foil-wrapped potatoes on a gas grill, too, but you may have to cook them longer (or heat the grill to high).
When you first try this method, you might want to use slightly shorter cooking times and check the potatoes (unfold the foil with tongs; the steam will be hot enough to burn). If they aren't quite done, rewrap them and grill for a few more minutes. If they're too charred but not done in the middle, put them back on, away from the coals, and cover the grill