PREHEAT grill to medium heat. Beat oil, mustard and vinegar with wire whisk until well blended. Season with salt and pepper. Remove 1/4 cup of the mustard mixture; set aside for later use.
TOSS potatoes and onions with remaining mustard mixture until evenly coated. Spoon onto greased double thickness of foil. Bring up foil sides. Double fold top and ends to seal packet, leaving room for heat circulation inside.
GRILL 30 to 35 min. or until potatoes are tender, turning packet over every 10 min. Open packet. Pour reserved mustard mixture over the potato mixture; mix lightly. Sprinkle with parsley.