Place 2 strips (approx. 45 cm each) of aluminium foil crosswise in a springform pan (28 cm Ø). Line the bottom of the mould with the foil, forming a rim all around.
For the caramel nuts, roughly chop the walnuts. Melt 40 g butter and pour it on the aluminium foil in the mould. Sprinkle 90 g sugar evenly on the butter. Then sprinkle the walnuts on top.
Wash the lemon hot, dab dry and finely grate the peel. Halve the lemon and squeeze it. Peel, quarter and core apples and cut them into fine slices. Mix apples with lemon juice and peel.
Cream 160 g butter, 200 g sugar and salt with the whisk of the mixer. Stir in the eggs one by one. Mix flour and baking powder and stir in.
Mix apples with 50 g sugar and spread on the nuts. Spread the dough on the apples. Bake in the preheated oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer) on the lowest rack for approx. 50 minutes.
Cover the cake with aluminium foil after about 40 minutes. Remove and leave to cool in the mould for at least 4 hours.
Turn the cake onto a plate so that the apple-nut layer is on top. Caramelise the layer carefully with a kitchen gas burner until golden brown and dust with icing sugar. Whipped cream tastes good with it.