Preheat the oven to 180°C bake. Generously butter the inside of the deep springform cake tin with butter, then dust with the caster sugar.
Cut the crusts off each slice of bread. Cut 15 of the slices in half making 30 long fingers. Cut the remaining 5 slices in half to make triangles. You should end up with 30 fingers and 10 triangles.
Use 20 of the fingers to line around the cake tin, overlapping the fingers and pressing down to seal as you go. Then place the triangles in a pretty pattern on the bottom of the tin. Set the remaining 10 fingers aside for the top. Transfer the cake tin to the fridge while you make the filling.
To make the filling, melt the butter in a large sauté pan over a medium heat, add the apples, then the sugar, toss to coat the apples. Add the orange zest and cinnamon stick. Sauté for 7–10 minutes until the apples are soft and the sauce has caramelised.
Discard the cinnamon stick. Spoon the caramel apple filling into the centre of the bread lined cake tin, top with enough pieces of bread to seal (these do not need to be pretty as they will form the base of the cake once flipped). Cover tightly with foil and rest a baking tray on top. Bake for 30 minutes, then remove from the oven and leave to rest for 60 minutes. Turn out onto a serving plate. Serve with custard.