01 -
Heat the grill to medium-high, about 400°F. Tear off 2 large sheets of heavy-duty foil, or use 4 sheets if doubling regular foil.
02 -
In a large bowl, toss the cubed potatoes, onion, butter, olive oil, garlic, parsley, salt, and pepper until the potatoes are evenly coated.
03 -
Divide the potato mixture between the foil sheets. Fold the foil over the potatoes and crimp the edges tightly, leaving a little space inside for steam.
04 -
Place the packets on the grill and cook for 25-30 minutes, shaking or turning once or twice, until the potatoes are fork-tender.
05 -
Carefully open the packets away from your face. Sprinkle cheddar and Parmesan over the hot potatoes, then loosely close the packets again.
06 -
Grill for 4-5 minutes more, until the cheese is melted and bubbling. Transfer to a serving dish, garnish with parsley or scallions, and serve hot.