For the filling, place the apples, butter, sugar and lemon juice and zest in a saucepan with a little water and cook gently until the apples have softened.
Put the apple mixture in the bottom of a pie dish, packing the apples down to provide a firm base for the topping (this helps it rise evenly during baking).
For the topping, sieve the flour together with a pinch of salt and the baking powder. In a separate bowl, cream together the butter and sugar, beat in the egg and then lightly fold in the flour. Add a little milk if the mixture is too stiff.
Pour the mixture on top of the apples and bake in the oven for 45-55 minutes or until the top is golden-brown and cooked through.