Cut the vanilla pod in half lengthwise. Scrape out the pulp. Wash lemon hot, grate dry and finely grate the peel. Peel and quarter apples, remove core. Cut quarter into pieces. Halve lemon.
Squeeze out one half and mix with the apple pieces. Use the rest of the lemon for other purposes.
Mix butter, 250 g sugar, lemon zest and vanilla pulp in a mixing bowl with the whisk of the hand mixer until whitish creamy. Separate the eggs. Stir the egg yolks individually into the butter mixture.
Stir in ground almonds and ricotta. Beat the egg whites with the whisks of the hand mixer until stiff, adding 100 g sugar. Add 1/3 of the egg whites and the apples to the dough.
Grease a springform pan (26 cm Ø) and dust with flour. Pour in the dough and sprinkle with almond flakes. Bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 1 1/4-1 1/2 hours.
Cover the cake with foil after about 45 minutes.
Switch off the oven and open one gap wide. Let the cake cool down in it. Dust the cake with icing sugar, cut into pieces and arrange on a plate.