This is an adaptation of a recipe I found while researching Georgian food for a party. It has become a favorite dessert for my group of friends. The original called for purple raisins, but I like the flavor of sultanas better. You can obviously use whichever you like best. I also think it might be good with some other dried fruits mixed in, but I haven't tried that yet.
Preheat oven to 350 degrees F (175 degrees C). Grease a 14x16-inch or larger baking sheet.
Spread walnuts out onto a baking sheet in a single layer and bake in the preheated oven until walnuts smell toasted and are lightly browned, about 10 minutes. Let cool; process nuts into tiny chunks in a food processor, pulsing several times to avoid turning them into nut butter.
Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add raisins to the steamer, cover, and steam until plump and moist, 10 to 15 minutes.
Combine walnuts, raisins, 1/2 cup white sugar, and cinnamon in a bowl.
Place 1 pie crust in the center of the prepared baking sheet. Cover with 1/3 of the raisin mixture, spreading it into a layer about 1/2-inch thick and leaving a 1-inch margin at the edge of the crust. Dot filling with 1 tablespoon butter chunks. Place 2nd crust over filling and spread 1/3 of the raisin filling as before; dot with 1 tablespoon butter. Repeat with 3rd crust and remaining 1/3 of raisin filling and remaining 1 tablespoon butter chunks. Place final crust on top and press edges together to make a dome with the filling sealed inside the crusts. Use your fingers to crimp the edges into a fluted shape.
Whisk egg with milk in a small bowl and brush the top of the pie with the mixture. Cut 5 2-inch-long slits into the top crust in a star pattern.
Bake in the preheated oven until golden brown, 50 to 60 minutes.
Combine 1/2 cup white sugar, brown sugar, and white grape juice in a small saucepan. Bring to a boil until mixture thickens into a syrup, 7 to 10 minutes. Drizzle syrup over the hot torte to make a glaze. Cool slightly before cutting into 16 slices for serving.