Blitz the oats in a food processor for a few minutes until fine textured. Add the wholemeal flour and sugar then mix again. Next, add the Stork and blitz until the mixture becomes like fine breadcrumbs. Finally, pour in the cold water, a tablespoon at a time, blitzing until the mixture becomes stiff. Roll the finished pastry into a ball, wrap up in cling film and leave to rest in the fridge for around 15 minutes.
Preheat the oven to 200c/ Gas 6/ 400F. Roll out the pastry roughly 5mm thick and use to line a flan case. Prick the bottom of the pastry with a fork, line with baking parchment and fill with baking beans. Bake blind for 15 minutes, then turn down the oven to 180c/ Gas4/ 350F, pour out the parchments and baking beans and cook for a further 15 minutes.
Remove the flan case from the oven and leave to cool down on a wire rack. Turn down the oven temperature again to 170c/ Gas3/ 325F.
Lightly beat the egg yolk with a whisk and add the plain flour, little by little, mixing thoroughly. Add the sugar, zest, maple syrup, milk, yoghurt, vanilla paste and salt.
Pour the mixture into the flan case and cook in the oven for about an hour. Cool the tart down completely and chill in a fridge for few hours or ideally overnight.