Preheat oven to 425°. Wash and dry potatoes and place in oven. Bake on a rack in the middle of the oven until done (approximately one hour). Remove potatoes from oven and reduce heat to 400°.
In a heavy skillet, fry bacon over moderate heat until golden brown. Drain the bacon on paper towels and set aside. Remove all but 2 TBS bacon grease from the pan. Add chopped onion and saute over low heat until translucent. Remove from heat. Cut a 1/4 inch slice off the top of each potato. With a spoon, scoop the pulp from the potatoes, leaving a shell approximately 1/4 inch thick. Place the pulp in a large bowl with the butter and 1/4 cup milk. Mash to a puree. Once the potato mixture is smooth, stir in cheddar cheese and sour cream, adding more milk, if necessary, to reach desired consistency (thick enough to mound into the shells). Stir in bacon pieces and sauteed onion and adjust seasoning. Spoon the potato mixture into the shells and place in a buttered glass dish just large enough to hold the potatoes. Bake until the tops are golden brown. Serve immediately. 6 servings.