Get the grill ready for medium heat or preheat your oven to 450°F. Cut each steak into 4 portions. In a large mixing bowl, toss together the steak pieces, potatoes, onion, garlic, rosemary, salt, and pepper.

Take eight 18x12-inch sheets of heavy-duty foil and place the mixture onto the dull side of each foil piece. Fold the foil over the mixture and seal the edges tightly to form packets.

Place the packets on the grill or in the oven. Cook for 25 minutes, turning them over halfway through, until the potatoes are soft. Open the packets cautiously to let the steam escape. If you like, add a sprinkle of additional rosemary before serving.