Tear off 36 x 18 piece of heavy foil. Fold in half to make a double thickness of foil. In large mixing bowl combine sliced potatoes, unpeeled garlic cloves, 1 T olive oil and salt; toss gently. Place potato mixture in the center of the foil. Top with rosemary sprigs. Bring up the two opposite edges of the foil and seal with a double fold. Fold remaining ends to completely enclose the potato mixture, leaving space for steam to build. Preheat gas grill. Place foil packet on grill over medium heat. Cover and grill about 35 minutes or until potatoes are tender. Discard reosemary sprigs. Meanwhile, in jar combine 1 T olive oil, vinegar, mustard and pepper. Squeeze the paste from the grilled garlic cloves into jar. Cover; shake well. Pour garlic mixture over grilled potatoes; toss gently to coat.
