To make the crust, add the flour, margarine, sugar and salt to a food processor. Pulse until the mixture resembles the texture of breadcrumbs. Add the ice water and pulse again until the dough stars to come together.
Turn out onto a work surface and use your hands to form the mixture into a ball. Cut off 1/3, form into a ball and wrap with cling film; this will be the lid for the pie. Form the remaining 2/3 of dough into a ball and wrap. Place the 2 balls in the freezer and allow to rest for 30 minutes.
Now make a start on preparing the filling. Peel, core and chop the apples into fine slices. Add these to a bowl along with remaining ingredients for the filling. Stir well to ensure the apple is well coated.
Pre heat the oven to 200°C / 180°C fan / gas 6.
Return to the pastry. On a lightly floured surface, roll out the larger ball of pastry until it is a couple of inches wider than the pie tin. Use your rolling pin to roll the dough over the tin. Press the pastry firmly into the edges and trim off the excess. Then with a fork poke holes in the base. Fill with parchment paper and baking beans. Bake for 10mins in the oven.
Whilst the pastry is blind baking, roll out the pastry lid and use the excess pastry to form leaf shapes. Remove the pie crust from the oven and fill with the apple mixture. Roll the lid over the top. Crimp the edges and decorate the top with the leaves.
Brush the lid of the pie with the oat milk, sprinkle over the sugar and finish by cutting 3 air vents into the pie.
Bake for 50 minutes at 180 fan. The pie is ready when the crust is a beautiful deep golden shade.
To make the vanilla custard slice the vanilla pod lengthways and scrape out the seeds with a knife.
Add the milk, sliced vanilla pod and seeds to a saucepan and heat until just below the boil. Then reduce the heat and remove the vanilla pod.
Meanwhile, in a bowl add 2 tbsp of oat milk, cornstarch, sugar, salt and turmeric. Whisk until it forms a porridge like consistency.
Pour the hot milk over the cornstarch mixture and whisk until smooth and silky.
Pour the custard back into the saucepan and gently heat until it has thickened to your desired consistency.