Grease a springform pan, ideally with a diameter of 24 cm. Preheat the oven to 180 ° C top / bottom heat.
Peel and core the apples and cut into approx. 1.5 cm thick wedges. Then place the apple wedges in a bowl and drizzle with the lemon juice.
Separate the eggs.
Put the egg white in a tall container and beat until stiff.
Add the sugar to the egg yolk in a bowl. Beat everything with the whisk of the mixer for about 1 minute until creamy. Add the spelled flour, baking powder, skyr and lemon zest. Then stir everything together until a creamy, lump-free dough is formed. Then fold in the egg whites, divided into 2 - 3 servings, with a spatula. Pour the finished dough into the greased pan and smooth it out a little. Then cover the top of the dough in a circle, starting from the outside with the apple wedges. Lightly press the apple wedges.
Put the apple pie in the oven and bake for 40-50 minutes until golden brown.
Remove the cake, let it cool and serve sprinkled with powdered sugar.