1 Place the puff pastry sheets next to each other on a board and defrost. Roll out very thinly to a circle about 8 ins (20 cm) in diameter.
2 For the caramel, heat the water and sugar in a saucepan and boil until deep amber in color. Remove immediately from the heat and stir in the butter. Pour immediately in the tart tin.
3 For the filling, peel, quarter, core the apples and slice into wedges. Wash and trim the rhubarb, then cut into 1 in (3 cm) pieces. Tightly pack the fruit in the tart tin, alternating between the apple slices and the rhubarb. Brush with melted butter and sprinkle sugar over the top.
4 Bake in the middle of a preheated oven at 375°F (190°C) for 35 - 40 minutes. Remove from the oven and place the puff pastry over the top. Press the sides in with a spoon handle. Bake for a further 30 minutes until golden brown. Remove from the oven and let cool slightly.
5 Turn out on a plate and serve with a few scoops of vanilla ice-cream.