Knead the flour, hazelnuts, sugar, fat and 3 tablespoons of water into a short pastry. Wrap in foil and put in a cool place for 1 hour. For the topping, wash the lemon thoroughly, dab dry and rub the peel.
Halve the lemon and squeeze the juice. Mix egg yolks, crème fraîche, sugar, vanillin sugar and lemon peel. Peel, halve and core the apples. Cut the apple halves several times about 1/2 cm deep.
Sprinkle with lemon juice. Roll out dough between 2 layers of foil (approx. 30 cm Ø). Put the dough into a springform pan (26 cm Ø) and press up about 3 cm at the edge of the springform pan. Prick the dough several times with a fork
Sprinkle evenly with ground hazelnuts. Place apple halves on top. Spread the egg-cream-fraîche around the apples. Sprinkle with nut leaves. Bake in the preheated oven (electric cooker: 200 °C/ gas: level 3) for about 40 minutes.
Heat the apricot jam and spread on the apples. Remove the cake from the tin and dust with icing sugar. Makes about 12 pieces.