Preheat the oven to 190C/375F/Gas 5.
Place the apples, lemon zest and juice, sugar and cinnamon into a heavy-based saucepan over a medium heat. Cover and bring to a simmer. Simmer, stirring frequently, until the apples break down and become soft.
Remove the lid, increase the heat to high and beat the apples vigorously until all the liquid has evaporated and the apples have formed a smooth purée.
Brush the slices of bread on both sides with the melted butter.
Lightly butter a 15cm/6in-diameter round cake tin and line the base and sides with three-quarters of the bread, overlapping the slices so that there are no gaps and making sure the bread comes slightly higher than the rim of the tin.
Spoon the apple purée into the tin and cover with the remaining slices of bread, overlapping the slices so that there are no gaps. Fold down the bread lining the sides of the tin and press down onto the bread lid to seal.
Transfer to the oven and bake for 30 minutes or until golden-brown. Remove from the oven and leave to cool slightly. Serve.
