Preheat oven 200C. Line a 20cm round cake tin with baking paper. In a large nonstick frying pan, melt the butter. Add the apple and cook, turning, until golden, about 4 minutes. Add the brown sugar and 2 tbsp. cold water. Cook, stirring until the sugar has dissolved and a delicious syrup has formed, 2 minutes. Bring to the boil, then reduce the heat to low and simmer for 2 to 3 minutes to thicken the syrup.
Arrange the apples in the cake pan. Drizzle that yummy syrup over the fruit. Cut a 22cm round from the sheet of puff pastry and place over the apples, tucking in at the edges.
Bake until the pastry is golden and puffed, about 20 minutes. Allow to stand for 2 minutes. Loosen the edges with a knife before carefully turning out the tart to serve.
4 tbsp (60g) butter
4 medium green apples, peeled, cored, then cutting 6 thick slices from each