Roll out the pastry on a floured surface and use it to line a 24cm loose-based tart tin. Cover with clingfilm and chill in the fridge for 20 minutes. Preheat the oven to 160°C/gas mark 3.
Wash and trim the rhubarb and cut into 3cm lengths. Place in an ovenproof dish, sprinkle with the granulated sugar and cover with foil. Bake for 15-20 minutes, until just tender but still holding its shape. Leave to cool completely. When the rhubarb is cold, strain it, reserving the juice.
Increase the oven temperature to 200°C/gas mark 6. Line the pastry case with baking parchment and fill with baking beans or uncooked rice. Bake blind for 18-20 minutes, removing the beans and baking parchment for the last 5 minutes. Set aside and reduce the oven temperature to 180°C/gas mark 4.
Meanwhile, cream the butter with the caster sugar in a bowl using an electric hand mixer. Add the almonds and orange zest, then the eggs, one at a time. Add the double cream and 100ml of the reserved rhubarb juice, and stir in gently.
Spoon the baked rhubarb over the pastry base and carefully pour over the cream mixture. Bake for 35-40 minutes, until set and lightly browned. Allow to cool then dust with icing sugar.