Par-boil the potatoes: Place the sliced potatoes in boiling salted water and cook for 5 minutes. Drain and transfer to a large bowl. Let them cool for about 10 minutes.

Prep the foil packets: Cut the steak into pieces according to your preferred doneness. In a large bowl, combine the steak, par-boiled potatoes, onion, olive oil, parsley, garlic, Italian seasoning, salt, and pepper. Mix well. Prepare 4 pieces of aluminum foil, each at least 12x12 inches. Divide the steak and potato mixture evenly among the foil sheets. Top with butter cubes. Fold the foil over the ingredients and seal tightly.

Cook the foil packets: On the grill: Cook the foil packets on a hot grill for 8-10 minutes per side until the potatoes are tender. Use an oven thermometer to check the steak's doneness. In the oven: Bake the foil packets at 425°F for approximately 20 minutes or until the steak reaches your desired doneness. Use an oven thermometer to monitor the steak's progress. For a char, broil the packets for 1-2 minutes at the end.