Preheat oven to 450F. Bake potatoes for one hour. When done, pull from oven and cool for 15 minutes. In a medium pan heat oil and cook onions until translucent. Cut a slit in the top of each potato, lengthwise. Open each potato and scoop it out into the pan with the onions, keep potato skin intact. Place each empty potato skin on a baking pan. Mash the removed insides of the potatoes in the pan with a fork. Add mustard & continue to mix and mash. Add Â¾ cup of the cheese and mix in. Fill each skin with the mixture, pinching the skin around the center, as if fluffing up a baked potato. Top each potato with remaining cheese & bake for 15 additional minutes. Serve with a dollop of sour cream. Excellent served with steak, pork, chicken, sausages or as a vegetarian entree.