Make the dough: In a small bowl, mix the yeast with the wine and brandy until dissolved; set aside.

Put the flour, butter, lemon zest and salt in the bowl of a stand mixer fitted with the dough hook and mix on low speed until well combined, with butter pieces no larger than a small pea, 3 to 5 minutes. Add the egg yolks and sugar and mix on medium-low speed until a shaggy dough forms, about 3 minutes.

With the mixer running, gradually pour in the yeast mixture. Once all the liquid is incorporated, increase the speed to medium and mix the dough until smooth and soft, 5 to 7 minutes.

Transfer the dough to a clean work surface and divide it into 4 equal pieces. Wrap each piece in plastic wrap and refrigerate for about 1 ½ hours. 

Make the poppy seed filling: Combine the ground poppy seeds, milk and sugar in a small saucepan and bring the mixture to a lively simmer over medium-high heat, then cook, stirring constantly, until the mixture becomes thick and spreadable, about 20 minutes. Remove the pan from the heat, stir in the vanilla, and set the filling aside to cool completely, about 30 minutes, stirring occasionally.

Make the walnut filling: Pulse the walnuts in a food processor until finely ground but not pasty. Transfer to a medium bowl, add the milk, sugar, lemon zest, and vanilla, and stir until combined. Set aside.

Make the apple filling: Using your hands, squeeze the juices out of the grated apples and then put them in a colander set on a plate to drain any remaining liquid. Set aside. 

Preheat the oven to 425°F. Butter a deep 9-inch square baking dish.

Make the glaze: Stir the apricot jam with the hot water in a small bowl until the jam is dissolved. Set aside.

Assemble the cake: Take one piece of dough from the refrigerator, lightly flour the work surface, and roll the dough out into a 10-inch square about ¼ inch thick. Measure the exact inside dimension of your baking dish and trim the dough square to that size. Gently roll the dough square up around your rolling pin, position it over the baking dish, and unroll it so it sits squarely in the bottom of the dish, trimming off any remaining excess dough if necessary. Reserve all the dough trimmings, covered loosely with plastic wrap so they don't dry out.

Spread the poppy seed filling in an even layer over the dough, making sure it comes all the way to the edges. Roll out a second piece of dough in the same manner. Trim it as necessary (reserve the trimmings) and place the dough over the poppy seed filling. Spread the walnut filling in an even layer over the dough.

Repeat the rolling and trimming with a third piece of dough and place it over the walnut filling. Sprinkle the ground walnuts evenly over the dough. Give the grated apples one more squeeze to remove any remaining liquid and distribute them evenly over the walnuts. Sprinkle the sugar and cinnamon over the apples.

Roll and trim the last piece of dough and place it over the apple layer. Brush it with 2 tablespoons of the apricot glaze. Poke holes evenly over the surface of the dough with a fork.

Gently press the reserved dough trimmings into a ball and knead a few times, until smooth. Roll out the dough to ¼ inch thick. Using a pastry wheel or a large knife, cut about 12 strips of dough that are about ½ inch wide and 11 inches long. Arrange the strips in a diagonal grip pattern over the top of the dough and trim the ends neatly. Brush the strips with the remaining apricot glaze.

Put the fluden in the preheated oven and immediately reduce the temperature to 375°F. Bake until the top is deep golden brown, 40 to 50 minutes. Remove from the oven and cool the fluden completely on a wire rack.

Run a sharp knife around the edges of the pastry to release it from the baking dish. Cut the fluden into 12 rectangular pieces. Use a small spatula to lift out the pieces; the first one might be a bit tricky. Serve the fluden at room temperature.