Grease a round springform cake tin with butter and dust with flour. Set aside.
Beat egg whites until stiff, cover and chill.
Cream egg yolk and sugar together, then stir in butter, lemon juice and lemon zest. Beat until batter is light and fluffy
In a separate bowl, sieve in flour, salt and baking powder. Mix to combine.
Fold egg whites and flour mixture into yolk batter. Pour batter into tin and smooth top out.
Preheat oven to 180 C / Gas 4.
Peel apples and cut in half horizontally. Remove core. Slice apples carefully, but do not cut all the way through – you need to leave an end piece to hold all the slices together.
Gently fan apple halves and arrange onto the batter. Sprinkle with sugar and bake in preheated oven for 40 minutes.
