Peel, quarter and core the apples. Cut quarters into pieces, mix with lemon juice. Grease a springform pan (26 cm Ø). Preheat the oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer).
Cut the vanilla pod lengthwise. Scratch out the pith. Place in a mixing bowl with 175 g butter, 250 g sugar and lemon zest. Stir with the whisks of the mixer until creamy. Separate the eggs. Stir the egg yolks into the butter-sugar-cream one after the other.
Stir in ricotta and ground almonds.
Beat the egg whites until stiff and add 100 g sugar. Approx. 1⁄3 Fold the beaten egg white into the dough. First fold in apples, then the rest of the beaten egg white. Fill into the mould and sprinkle with almond flakes.
Bake in the hot oven 1 1⁄4-1 1⁄2 for hours, covering with aluminium foil after about 45 minutes.
Switch off the oven. Let the cake cool down for about 30 minutes with the oven door slightly open. Take out, let it cool down and dust with icing sugar.