Scrub the potatoes and prick them on both sides with a fork. Lay them on the oven rack and bake them for 1 1/4 hours or until they are soft.
Cut the potatoes in half lengthways. Scoop out the middles and mash them with the butter or margarine. Mix the apple and onion into the potato.
Put the potato shells in a heatproof dish. Pile the stuffing mixture inside them and make ridges on the tops with a fork. Return the potatoes to the oven for 15 minutes or until the tops are beginning to brown.