Tear off a 36x18-inch piece of heavy foil; fold in half to make an 18-inch square. In a small bowl stir together butter, the snipped or dried basil, the lemon-pepper seasoning, and garlic. Set aside one-quarter of the basil-butter mixture.
Place sliced potatoes and onions in center of foil; dot with remaining basil-butter mixture. Bring up opposite edges of foil and seal with a double fold. Fold remaining edges together to completely enclose vegetables, leaving space for steam to build.
Place foil pack on the grill rack directly over medium-hot coals; grill about 35 minutes or until potatoes are tender.
Meanwhile, trim fat from steak. Add steak to grill over medium-hot coals for the last 14 to 18 minutes of grilling for medium-rare doneness (145 degree F) or the last 18 to 22 minutes for medium doneness (160 degree F); turn steak once halfway through grilling.
Transfer steak to a serving platter; dot with the reserved basil-butter mixture. Slice across the grain; season to taste with salt and pepper. Serve with potatoes and onions. If desired, garnish with basil sprigs. Makes 4 servings.