Heat the oven to 190C (170C fan) and butter a 1 litre pudding basin or a 20cm round cake tin well, then set aside.
Cut the bread into fingers roughly 4cm wide. Dip each one quickly in the melted butter on both sides and line the base and sides of the tin, overlapping the pieces so there are no gaps. Keep a few pieces back for the lid.
Put the apples in a pan with the sugar, cinnamon if using, and the water. Cook over a medium heat for 8 to 10 minutes until just soft but still holding some shape. Taste and add a little more sugar if your apples are very sharp, then cool for 5 minutes.
Spoon the apples into the bread-lined tin, packing them down lightly, and pour over any juices. Cover the top with the remaining buttered bread, pressing the edges together to seal.
Bake for 30 to 35 minutes until the bread is deep golden and crisp and you can hear a faint bubble of juice underneath.
Rest for 5 minutes, then run a knife around the edge and turn the charlotte upside down onto a serving plate in one confident flip. Serve warm with custard or a splash of cream.