Line a springform pan with greased parchment paper and place the peeled apples cut into eighths and the halved walnuts on top. Sprinkle with 90 g of sugar.
Mix the butter with 100 g sugar and the grated lemon zest for 8-10 minutes until frothy. Gradually add the cornstarch, sifted spoonfuls of cornstarch, flour and baking powder, the eggs, as well as milk and salt. Stir vigorously, then pour the batter over the apples and bake the cake over moderate heat (170 ° C) for 45-55 minutes (chopsticks test). Leave switched off in the pipe for another 10 minutes.
Take out, let cool for 10 minutes. Loosen the edge of the springform pan, turn the cake over and remove the cake tray and paper from the cake.