Cream butter and part of the sugar until creamy. Whisk in egg. Zest lemon and add half of the zest to the butter mixture. Add flour and salt, mix until smooth. Chill for approx. 30 min.
Pre-heat the oven to 180°C/350°F. Roll out dough. If necessary, add extra flour.
Put dough in a pie dish lined with parchment paper. Pierce dough all over with a fork.
Spread hazelnuts on top.
Whisk together remaining lemon zest with part of the sugar, eggs, and cream.
Peel, core, and dice apples. Distribute over the base and pour over the cream mixture.
Bake at 180°C/350°F for approx. 40 min.