Preheat the oven to 160 C, fan 140C, gas 3.
In a bowl, combine the biscuits, melted butter, zest of 1 of the oranges and the mixed spice. Press into the base of a loose bottomed 23cm round cake tin.
In a separate bowl, whisk the soft cheese, ricotta, sugar and remaining orange zest with a hand-held electric mixer until smooth. Stir in the Grand Marnier, if using until combined. Add the eggs and whisk for a further few seconds.
Scatter half the mince pies over the biscuit base, then top with half the cheese mixture. Repeat.
Bake for 1 hour until just set. Remove from the oven and leave to cool in the tin before cutting into wedges.
Serve decorated with orange zest and a dollop or two of lightly whipped cream.