Prepare grill for medium heat or preheat oven to 450°. Cut each steak into 4 pieces, for a total of 8 pieces. In a large bowl, combine steak, potatoes, onion, rosemary, garlic, salt and pepper. , Divide mixture among eight 18x12-in. pieces of heavy-duty foil, placing food on dull side of foil. Fold foil around potato mixture, sealing tightly. , Place packets on grill or in oven; cook until potatoes are tender, 8-10 minutes on each side. Open packets carefully to allow steam to escape. If desired, sprinkle with additional rosemary.