PREHEAT oven to 400°F. Bake potatoes 1 hour 15 minutes or until tender. Cut potatoes in half lengthwise. Scoop out inside of each potato half, leaving thin shell. Place potato flesh in medium bowl; mash with electric mixer or hand masher until smooth. Set aside.
COOK and stir onion in margarine in small skillet until tender; stir in mustard. Add to potatoes along with 1/2 cup of the cheese and the pepper. Beat with electric mixer on medium speed until well blended. Pipe or spoon potato mixture into potato shells; sprinkle with remaining 1/2 cup cheese.
BAKE an additional 20 minutes or until potato filling is heated through and cheese is melted.