Preheat the oven to 180C/160C Fan/Gas 4.
Peel, core and roughly chop the apples. Add the apples to a saucepan with the lemon juice and water. Stir, cover and cook briskly for five minutes until the apples are soft.
Add the butter and caster sugar and stir. Then transfer to a 900ml/1½ pint capacity ceramic gratin dish, about 5cm/2in deep. Leave to cool while you prepare the topping.
For the topping, cream together the butter and caster sugar until fluffy and light.
Fold the flour and egg in alternate spoonfuls into the sugar mixture until blended, being careful to keep folding rather than stirring energetically - this will keep air in the mixture and fold in. Add a spoonful of boiling water to the mix.
Spoon the mixture over the apples. Cook in the oven for 30-35 minutes or until the topping is puffy and golden. Serve with cream or custard.