Quarter the apples, finely slice them crosswise. Add the sugar, sultanas, rum and cinnamon, mix together well.
Unfold the pastry sheets, place one sheet of pastry on a baking tray lined with baking paper (approx. 30 cm in diameter), brush with a little butter. Place two slightly overlapping pastry sheets over the top, brush with a little butter. Spread the hazelnuts and filling on top, fold the overhanging pastry edges loosely over the filling. Cover with the final sheet of pastry, tuck under any corners, brush with the remaining butter and sprinkle with sugar.
for approx. 20 mins. in the bottom half of an oven preheated to 200°C.
Serving suggestion: custard or vanilla ice cream.