Heat the oven to 200C/400F/Gas 6.

Scrub the potatoes and with a fork prick each one in 2 places on both, sides. Bake the potatoes in the oven for 1 1/2 hours.

Remove the potatoes from the oven (leaving the oven on), allow to cool slightly, then cut each one in half lengthways. Scoop the cooked potato into a bowl, leaving the shells intact. Mash the potato well. Peel, core and finely chop the apple.

Melt the margarine in a small frying-pan, add the onion and fry gently until it begins to soften, stirring occasionally. Stir in the apple and cook for a further 2 - 3 minutes, until soft.

Stir the apple and onion mixture into the mashed potato. Add the sage, mustard and a little salt. Mix thoroughly.

Spoon the mixture back into the potato shells and make criss-cross patterns on the top with a fork for a decorative finish.

Put the potato shells in a greased shallow ovenproof dish and return to the oven. Bake for 15 minutes until the tops are browned. Serve piping hot. Alternatively, top the potatoes with grated cheese