Cut your apples into thin slices. In a large skillet, combine the apples, sugar, cinnamon, salt, raisins, and apple cider vinegar. Cook over medium heat, stirring frequently, until the liquid from the apples is all gone and the apples are soft, 10 to 15 minutes. Stir in the chopped walnuts, and set aside to cool.
Preheat the oven to 350°F and set an oven rack in the middle position. Line a 13 x 18-inch baking sheet with parchment paper.
Melt the unsalted butter and set aside. Unroll the phyllo dough sheets and keep it covered with a damp paper towel so it doesn’t dry out while you work with it.
Carefully place one phyllo sheet on a clean work surface or in the baking sheet itself. Brush the sheet with the melted butter until the top is completely covered. Layer with four more phyllo sheets, brushing each layer with the melted butter so you have a nice stack of five sheets.
Next, spread the panko bread crumbs over the topmost phyllo sheet, followed by the apple mixture on top. Leave about 2 inches of space all around your apple mixture to prevent the filling from spilling out. Fold the sides of the phyllo over the filling, then fold the bottom edge of the phyllo over the filling. Roll up jelly-roll style and carefully transfer to the prepared baking sheet, seam side down.
Brush the top of the strudel with the remaining butter. Cut diagonal slits through the top layers of the phyllo dough—not all the way, just deep enough until you reach the filling.
Pop the baking sheet into the oven and bake the strudel until it’s golden, around 40-50 minutes. Let cool. Dust with powdered sugar and serve warm!