Preheat your oven to 180°C (360°F).
Line the bottom of a cake pan with baking paper.
Peel the apples and slice them thinly. Pour lemon juice on the apple slices to avoid oxidation.
In a saucepan, melt the sugar over low heat and stir a little. Remove from the heat as soon as it starts coloring. Pour the caramel in the cake pan.
Halve the hazelnuts. Place them face down in the caramel. Add the apple slices on top. Set aside.
In a mixing bowl, mix the eggs and sugar. Add the butter, flour, baking powder, and salt. Stir until smooth.
Pour over the hazelnuts and apples.
Bake for 40 minutes.
To remove from the cake pan, flip the cake upside down onto a plate.