Heat the sugar over a high heat in a saucepan until it turns a golden color and starts to foam. Add the butter and mix until it melts with wooden spoon. Should take 3-4 minutes all told. Pour the caramel into an 8″ metal baking dish, split the vanilla pod into two and place in pan forming a letter “V”. Having set your oven to 325F (160C) peel and core the apples.
Cut the apples vertically and try to get an equal thicknesses so they cook evenly. Arrange them in circle in the the baking dish and ensure even coverage and thickness filling any gaps. Some people use larger slices but if you want more of your apple to go brown then cut them into thinner slices.
Place the dish on a lower rack in the oven and cook for about an hour, ensuring they are all evenly cooked. Allow to rest and then chill for an hour. Next up – cook the pastry. Roll out your pastry on a sheet of parchment/greaseproof paper and using another dish of the same size as a template, trim the pastry to the right size and shape. Leave the paper slightly larger – one inch margin all round will be perfect.
Prick the surface of the pastry disc. Slide the paper and pastry onto a baking tray and cook the pastry in a preheated oven at 400F (200C) for about 10 minutes. it will still be slightly soft but will harden as it cools.
The final step requires some confidence and dexterity. The pastry disc is placed on top of the caramelised apples in the baking dish and the dish rotated to ensure the apples are not stuck to the pan. A plate is then placed over the top of the pan and whole dish inverted.