Roll out pastry on a floured board. Line a 23cm glass pie dish. Trim pastry, leaving a 1cm overhang, and crimp edge with a fork. Prick the bottom and side of case all over with a fork. Chill case until firm, about 30 minutes. Line pastry case with parchment and fill with dried beans.
Meanwhile, combine sweet potato, milk, double cream, caster sugar, eggs, vanilla, ground ginger, salt and nutmeg in a liquidiser. Pulse until smooth. Fold in crystallised ginger. Set aside.
Bake pastry in the preheated oven for 25 minutes. Carefully remove parchment and beans and return to oven. Bake until just golden, about 10 minutes more. Pour sweet potato filling over pastry and bake for 40 minutes.
Combine 100g caster sugar and egg whites in a metal bowl set over a pan of simmering water, ensuring the bowl does not touch the water. Stir until sugar is dissolved. Remove bowl from heat and beat whites until stiff. Mound meringue in centre of hot filling and spread to edge of pie.
Bake in preheated oven until meringue is golden brown, 12 to 15 additional minutes. Sprinkle with cinnamon.