Heat water and sugar in an oven-safe casserole and make a golden-brown caramel. Take off the flame, add butter and stir thoroughly.
Peel the apples and cut into thin slices (about 5mm thin).
Arrange the apples onto the caramel circularly in the casserole (careful, the caramel is very hot!).
Roll the puff pastry out very thin, cut out a circle that’s slightly bigger than the casserole and cover the apples with it. With your fingers, press the pastry rim onto the casserole.
Bake at 170°C for about 30 minutes until the puff pastry is golden brown. Take out of the oven and let cool down for about 15 minutes before turning the tarte over. Serve with crème fraîche.