Preheat the oven to 190°C/gas mark 5. Cut the pastry into a 22cm circle and press into a round 18cm loose-based cake tin, lining the base and part way up the sides.
Using a handheld electric whisk, cream the butter and sugar together. Add the eggs and 2 tbsp ground almonds, and whisk again. Fold in the remaining ground almonds with the orange zest and cinnamon.
Spoon the almond mixture into the pastry-lined tin, then top with the rhubarb. Sprinkle over the flaked almonds.
Bake for 40 minutes, until golden. Dust with icing sugar and remove from the tin. Cut into slices and serve warm.