Preheat the oven to 180C/Gas 4.
Place the chopped apples in the saucepan with the lemon juice and water.
Stir, cover and cook briskly for 5 minutes until the apples are soft.
Add the butter and caster sugar and stir. You should end up with a slightly lumpy purée.
Transfer the apple mixture to the gratin dish and leave to cool.
Place the butter and caster sugar in the large mixing bowl and beat together with the electric hand whisk until light and fluffy.
Beat in the eggs, one at a time and then fold in the flour.
Add the boiling water and mix thoroughly.
Spoon the mixture over the apples and use a palette knife to spread it evenly.
Place in the preheated oven and bake for 30-35 minutes or until the topping is golden. Check the pudding after 20 minutes to make sure the top isn’t burning. If it is, place a piece of kitchen foil over the top.
Remove from the oven and serve immediately with cream or custard.
