Scrubb potatoes, dry, then pierce with fork several times. Arrange on a baking sheet and bake in 375 degree oven for about 1 hour or until tender. Let stand until cook enough to handle.
Cut potatoes in half lengthwise and scoop out centers, leaving 1/8" potato in shell. Cover. Chill shells up to 2 days until ready to use for the skins.
If desired, add the 1/3 cup milk, 1/4 cup butter, salt & pepper to use potato centers as mashed potatoes.
When ready to prepare skins, melt the 1/3 cup butter. Brush shells with the butter, inside and out. Place, cut side up, on baking sheet. Bake in a 500 degree oven for 20 minutes or until brown and crisp.
Distribute some bacon, onion, and cheese in each shell. Bake just until cheese melts. Serve hot. If the potatoes are large, you may want to cut in half crosswise before serving.
You may want to double this recipe if you are serving several people, especially if they are hungry!