Preheat oven to 400 degrees. Wash potatoes, drying the skins with some paper towels. After drying, lightly oil the skins of potatoes with vegetable oil, and wrap them each with aluminum foil or for a crispier skin, don't wrap them. Place potatoes apart (about 4-6 inches), on cookie sheet for even baking. Bake in oven for approximately 45 minutes to an hour.
When done, take potatoes out and split all potatoes lengthwise down the middle, LEAVING THE OVEN ON. (Potatoes will be returned to the oven, once stuffed). Be careful and use a pot holder because the potatoes will be extremely HOT. Cut each potato lengthwise and squeeze (vertically and horizontally) so it will be easy to scoop the pulp out of each potato, leaving about 1/4 layer of potato in the skin and place pulp into a mixing bowl. Add milk (just enough to moisten potato mixture), butter (enough to make creamy), mayonnaise or Miracle Whip, salt, pepper, then add and fold in cheddar cheese and onion. Mix this mixture until very creamy, but still thick. Once mixed to your desire, scoop the mixture back into each potato skin, distributing the potato mixture evenly into each potato.
Place the cookie sheet with the stuffed potatoes back into the heated oven for about 20 minutes and the stuffing is a little bit browned on the top. Then remove the potatoes and get READY TO EAT!!!!!