1. Peel potatoes. Cut each crosswise into 1/4-inch-thick slices. Cut onion into 1/4-inch-thick slices.
2. Prepare outdoor grill with hot coals arranged for indirect grilling, or heat gas grill for indirect grilling.
3. In large bowl, combine melted butter, oil, mustard, salt and pepper. Add potato and onion slices; toss to evenly coat. Place in regular-size aluminum foil packet; or place on large sheet of heavy-duty foil, and then fold to make packet.
4. Grill over indirect heat, covered, for 1 hour or until tender.
5. Carefully open packet and place potatoes and onions on serving platter. Makes 4 servings.