Preheat the oven to 180C/350F/gas 4.
Place the apples, caster sugar and butter in the large saucepan with a little water and bring to the boil.
Cover and cook on a high heat until the apples are soft.
Remove the lid and stir the apple mixture until you have a thick, dark golden apple purée. You will need to keep stirring and adjusting the heat to avoid it catching and burning at the bottom of the pan. Remove from the heat.
Add the lemon zest, stir in thoroughly and set aside.
Place the clarified butter or ghee in the small saucepan and heat until melted.
Cut a piece of bread into a round that exactly fits the bottom of the charlotte mould. You may need to cut the bread lengthways to get a slice big enough to get the right dimensions. Brush on both sides with the melted butter or ghee and place it in the bottom of the mould.
Cut enough oblongs of bread, about 4-5cm in width and the length of the sides of the mould.
Brush the oblongs of bread with the melted butter or ghee on both sides.
Arrange the oblongs around the sides of the mould, overlapping by at least 1cm as you go to ensure no apple mixture seeps out.
Fill the cavity with the apple mixture and then place enough sliced, buttered bread on the top to make a cover that fits the top of the mould exactly.
Cover the top of the mould with kitchen foil and place on a baking sheet.
Place in the oven and bake for 20 minutes.
After 20 minutes remove the kitchen foil and bake for a further 10-15 minutes or until the top is golden and crisp.
Remove from the oven and allow to cool slightly before turning it out onto a serving dish.
