To make the pastry:
In the processor, mix the butter, icing sugar and egg together. Add in the flour. Gather up the pastry and chill for an hour.
To make the filling:
Also in the processor, mix the butter, sugar, biscuits, ground almonds and eggs.
Set the oven to Gas Mark 4, 180C (350F). Roll out the pastry to line a 28cm diameter x 3cm deep tart tin. Chill for 10 minutes. Spread the filling mixture over the base. Place the plum slices on top and sprinkle with an extra tablesp. sugar. Bake for 45-50 minutes until completely set and nicely browned.