Combine gluten-free flour (available from health food shops, make sure it contains xantum gum) and butter together in a large bowl until they resemble breadcrumbs. Using a fork, add the whole egg and mix until a soft dough becomes sticky.
Turn out onto lightly floured surface – again gluten-free – and knead for two minutes. Wrap in cling wrap and put into the fridge for 30minutes to 1 hour. Grease an 11” tart tin with margarine & flour (gluten-free) lightly
Roll out pastry and line tin.
Prick pastry with a fork and return to fridge for 20 minutes. Preheat oven to 180°C. Line pastry case with tinfoil and fill with dried bean or short grain dried rice and bake blind until golden – about 15 to 20 mins.
Remove from the oven and allow the pastry to cool slightly. Remove the beans and foil and brush the baked pastry with egg white.
Return to the oven for 5 mins. This seals the pastry case and keeps it crispy.
Cut rhubarb into 2” pieces. Place them in an oven proof baking dish, and pour apple juice over them. Cover with tinfoil and cook in the oven for 15 mins at 180°C.
Drain the apple juice and arrange the rhubarb in the pastry case with the vanilla custard. Split vanilla lengthwise and scrape out the seeds
Beat the egg yolks, sugar, vanilla seeds and vanilla extract until just mixed
Add the cream and lemon zest and mix thoroughly.
Gently pour the vanilla custard filling over the rhubarb. Bake at 170°C for 30-40 minutes until custard is set. Allow to cool completely.