Cut the apples in half, peel, core and slice to an even sizes.
Place the butter in a tarte Tatin pan (or oven-proof frying pan) and press it out to cover the bottom. Sprinkle with the sugar and set aside. Preheat the oven to 180°C/gas mark 4.
Place the pan on the stove, and melt the butter and sugar together until they start to brown. This should take approximately 10 minutes.
Roll the puff pastry out to fit the tarte Tatin pan and until it is around 3mm in thickness. Place 7 pieces of apple, cut side up, around the pan containing the butter and sugar mixture, placing 1 in the centre. Cover with the pastry disc and tuck the edges in.
Finish cooking the tarte in the oven for 45-50 minutes. Remove and leave to rest for 10 minutes, then turn the Tatin out by placing a plate upside down over the entire pan and flipping the whole thing over.