Melt the butter in a saute pan. Add the apples, cover and cook for 10 minutes. Take off the lid, sprinkle over the granulated sugar, toss in the raisins, if using, and cook until the sugar caramelizes and coats the fruit (by now possibly a shaggy, golden mass, depending on the type of apples). Taste. Depending on the sweetness of the fruit, you may want a squirt of lemon for balance.
Roll the pastry out to a large square, about 1/4-inch/5 mm thick. Cut into 4 squares, then roll each one to make it slightly rectangular (not that shape matters one bit). Spoon the fruit mixture over half of each pastry, leaving a 1 cm/5 mm margin. Brush the margins with the egg wash. Fold the pastry over the filling and press the edges to seal. Set on a damp baking sheet. Slash the top of each pastry with a knife. Brush with more egg wash and let rest in the refrigerator for 15 minutes.
Preheat the oven to 425 degrees F/220 degrees C.
Bake the pastries for 10 minutes. Lower the oven temperature to 375 degrees F/190 degrees C and continue baking until golden, puffed up and brown, 10 to 15 minutes longer. Cool slightly on a wire rack and serve dusted with confectioners' sugar.
Recipe courtesy of Laura Calder