Scrub potatoes and pat dry. Prick skin of each with a fork. Bake in 400 degree oven 1 hour or until soft and skins firm. Cut each in half longways.
Carefully, using a spoon, scoop out the pulp, leaving enough for a 1/4 inch thick shell. Put potato pulp in a bowl and mash, beat in enough milk to make smooth and fluffy. Add butter, egg, onions, salt and pepper. Spoon back into shells, mounding slightly.
Sprinkle with bacon, top with a layer of cheese. Place in 350 degree oven to just heat through (about 10 minutes). Serve Immediately.