Preheat oven to 180C.
Place your tarte tartin dish (or 24cm oven proof frying pan) over a medium heat and melt the butter (don’t let it brown). Add the sugar and swirl around a little. Remove from the heat and arrange apple slices in your pan – think about finished presentation when you’re arranging, don’t forget that you will be inverting this onto your platter so presentation side will be the bottom). Sprinkle over the lemon zest.
Place the pan back over the heat and cook apples until they start to caramelise. This may take 20 minutes or so. You should be able to smell the caramel starting and the apples will shrink a bit. You want them to become a lovely caramelly colour – if you don’t get them dark enough at this stage, the tarte will just be sweet without that beautiful deep toffee flavour.
Cut the pastry into a circle about 2cm bigger than your pan and dock (prick it all over with a fork so that it doesn’t puff up too much in the oven). Lay it over the apples and tuck the sides in around the apples. Press down lightly. Bake in your preheated oven for 25-30 minutes.
Remove from the oven and allow to cool slightly then turn onto a serving plate and serve warm (not hot) with cream or ice cream.