Tear 2 squares of heavy-duty aluminum foil, approximately 20 by 20 inches, and lay them side by side and flat on a work surface. Divide the potatoes and layer them evenly on each piece of foil with the pancetta, sage, salt and pepper. Fold up the sides slightly to form a bowl shape. Drizzle with the olive oil (about 2 tablespoon per pouch). Secure the tops of the pouches by folding over the foil to seal in the steam and keep the olive oil from leaking out.
Cook on the shelf of the grill for 20 minutes, or over direct heat for 10 minutes and indirect heat (with half of the burners ignited on the grill or coals pushed to one side) for another 10 minutes. When done, the potatoes should be tender and cooked through, with some of them beginning to caramelize on the bottom of the pouch.
Being careful of the steam inside the pouch, open the pouch and transfer to a serving vessel--or serve from the aluminum foil.
This is a cooking method that you can do in the oven or on a campfire as well as a grill. Try roasting other vegetables in cartoccio on the grill.