HEAT grill to medium heat.
BEAT first 4 ingredients with whisk until blended. Reserve 1/4 cup mustard mixture for later use.
TOSS potatoes and onions with remaining mustard mixture until evenly coated. Spoon onto center of large sheet heavy-duty foil sprayed with cooking spray; fold to make packet.
GRILL 30 to 35 min. or until potatoes are tender, turning packet over every 10 min. Open packet. Pour reserved mustard mixture over potato mixture; mix lightly. Sprinkle with parsley.