Step 1

Prepare a grill for medium-high level indirect heat: For gas grills (with 3 or more burners), turn all the burners to medium-high heat; after about 15 minutes, turn off one of the middle burners and leave the remaining burners at medium high. For charcoal grills, bank one chimney starter-full of lit and ashed-over charcoal briquettes to one side of the grill. Set up a drip pan on the other side to avoid flare-ups. Replenish the charcoal as needed. (Be sure to consult the grill manufacturer's guide for best results.)

Step 2

Peel and cut the sweet potatoes into 2-inch chunks, then place them into a large bowl and set aside. Add the olive oil, vinegar, scallions, garlic, cilantro, 1 teaspoon salt and a generous grind of pepper to a blender. Blend until smooth. Reserve 1/3 cup of the sauce and pour the remaining over top of the sweet potatoes and toss until well coated.

Step 3

Cut four 22-inch long pieces of heavy-duty foil. Place two sheets on top of each other, then add half of the potatoes to the center. Fold the foil up and over the potatoes and seal tightly. Repeat with the remaining two pieces of foil and potatoes.

Step 4

Place the foil packs on the direct-heat side of the grill and grill for 10 minutes (do not open the foil packs—the potatoes need the steam to cook properly). Transfer to the indirect-heat side of the grill and continue cooking, turning the packs occasionally with grilling tongs, until the potatoes can easily be pierced with a fork, 25 to 30 minutes. Transfer to a platter and serve with the remaining chimichurri.