Mix the apples, sugar, cinnamon and apple juice in the prepared dish. Spread the butter over the top.
Approx. 15 mins. in the centre of an oven preheated to 200°C.
Beat the egg yolks and sugar for approx. 5 mins. using the whisk on a hand mixer. Add the lemon zest and juice with the vanilla sugar and flour, mix. Beat the egg whites with the salt until stiff. Add the sugar, beat briefly, fold into the egg yolk mixture using a rubber spatula, spread on top of the apples.
Bake for approx. 10 mins., remove from the oven, dust with icing sugar.