Put all the ingredients in a large bowl and mix until smooth.
Preheat the oven at 180°C (275°F), static.
Pour the batter in a mold, greased with batter (our baking tray was 30x11 cm).
Core, half and thinly slice the apple.
Roughly grind the hazelnuts.
Top with the apple slices and the hazelnuts, then sprinkle with sugar.
Bake for about 40 minutes.