Preheat oven to 200C/180C fan/gas 6.
To make the pastry, sift the flour into a large mixing bowl and add the butter and margarine. Using your fingers, mix together until the mixture resembles breadcrumbs.
Add about 5-6 tbsp cold water – 1 tbsp at a time – to bind the mixture into a ball. Then wrap it in cling film and leave to chill in the fridge while you prepare the apples.
While the pastry is chilling, core the apples, then cut into even-sized chunks so they all cook in the same amount of time.
Toss the sugar, lemon juice and cornflour into a bowl with the apples and combine.
Take the pastry from the fridge, Divide it into 2 portions, one slightly larger than the other, then roll out the larger piece until it's about 30 cm in diameter, gently pressing into the corners of the pie dish or plate.
Brush the edge of the pastry with a little milk and then pile the apples into the lined pie dish.
Roll out the second piece of pastry into a circle, use to cover the apples. Press the edges together to seal, then use a sharp knife to cut away any excess.
Brush the entire top of the pastry with milk and sprinkle over a little sugar.
Bake in the preheated oven until the apples are tender, approx. 45 minutes to 1 hour.