Preheat the oven to 180'C.
Scrape the seeds from the vanilla pod, cut the butter into small cubes and place in a bowl. Add the flour and the sugar and rub together with the tips of your fingers until the mixture resembles breadcrumbs. Make a well in the middle of the bowl, whisk the egg yolk and water together and add to the well. Bring the mix together until it forms a ball. Knead with the palm of your hand until the dough begins to come together, wrap in cling film and rest in the fridge for at least 30 minutes.
Roll the pastry out between 2 sheets of baking paper to 5mm thick. Cut out using a large cutter.
Transfer the pastry discs to the tart case. Line the inside of the tart with baking parchment or cling film and cover with baking beans. Transfer to the oven and blind bake for 10 minutes until starting to turn golden brown. Once cooked, check the pastry case for any small cracks and fill with any left over pastry if necessary. Brush the pastry with a little beaten egg and return to the oven for 5 minutes.
When the tarts are blind baked, remove from the oven and allow to cool.
Peel and remove the core from the apples. Dice them roughly into 1cm pieces. Cut the vanilla pod down the middle and scrape out the seeds. Place the vanilla seeds, pod, sugar and water into a saucepan along with the apples and cook over a medium heat. They should only be on the heat for 4-5 minutes. Once cooked transfer the apples from the saucepan into a bowl. You do not want the apples to remain in the hot pan as they will overcook very quickly and turn to mush. Allow to cool for 10 minutes before filling into the blind baked tart cases.
Preheat oven to 180'C. Line a baking tray with parchment paper, or use an Exopat mat.
Toast the hazelnuts in a dry pan for 2-3 minutes, crush in a pestle and mortar but not to a fine powder so that you keep some texture.
Whisk the butter and sugar until pale and fold into the flour until well incorporated. Fold in the crushed hazelnuts. Add the oats and gently fold together.
Transfer to the baking sheet and bake for 15 minutes. Remove from the oven and stir to break up the crumble. Return to the oven and carry on cooking until golden brown. Allow to cool.
Put a generous amount of crumble topping on the apple and return to the oven for 5 minutes.
This will allow the crumble topping touching the apples to soften while the crumble mix on top remains crunchy.
Remove from the oven and finish with a dusting of icing sugar.
Serve hot or cold.
«Vary the flavour of this basic apple pie by adding spices such as cinnamon and mixed spices.»