Heat oven to 200C/fan 180C/gas 6. Line two baking sheets with baking parchment. Unroll one of the pastry sheets onto a lightly floured surface, with the long edge nearest you. Spread with mincemeat, top with the second sheet of pastry, then cut in half, widthways, down the middle.
For each piece, cut into 4 strips down, measuring 6cm, 4cm, 2cm, and 1cm in width. Next, cut across evenly to make 12 ‘ladders’ of pastry. Dip the knife in flour if it sticks. Brush with egg. Chill for 10 mins.
Build the trees on the baking sheets. Stack the strips, mincemeat middles facing up, starting with the longest and finishing with the smallest. It helps them look more tree- like if you fold them a little in the middle, too. Once stacked, squash with your hand to flatten into a tree shape. Can be frozen for up to a month. Bake for 20 mins (25 mins from frozen) or until the pastry is golden and the mincemeat bubbling. Leave to cool on the tray for 5 mins, then lift onto a wire rack with a palette knife and let cool completely. Dust with icing sugar to serve.