Add the all-purpose flour, icing sugar, cold cubed unsalted butter, and lemon zest to a mixing bowl. Rub the ingredients together with your fingertips until sandy and crumbly.

Add the 4 egg yolks and salt to the crumbly mixture. Mix only until a dough forms.

Shape the dough into a flat disk. Chill it for at least 60 min.

Roll out about one-third of the chilled dough on baking paper. Cut it into strips and weave the strips into a lattice.

Bake the lattice on baking paper at 170°C / 340°F for about 20 min. Remove it when lightly golden.

Peel and cut the 600 g apples into small cubes. Toss them with lemon juice, granulated sugar, and ground cinnamon.

Cook the seasoned apples in a saucepan for 5 min to soften them. Stir in the roughly chopped walnuts.

Roll out the remaining dough and line the 20 cm tart pan. Press the dough into the base and up the sides.

Crumble the dry biscuits evenly over the pastry base. Spread the cooked apple and walnut filling over the biscuit layer.

Cover the tart with aluminum foil and bake at 170°C / 340°F for about 40 min. Remove the foil during the final 5 min if the pastry needs more color.

Place the pre-baked lattice on top of the cooled crostata. Rest the crostata overnight before serving.