Preheat oven to 235°C (455°F) with the rack positioned in the middle. Line a baking sheet with parchment paper. Heat apple cider vinegar in a small saucepan over medium heat until it boils. Add the golden raisins, cover, and remove from heat.
Heat 1 tablespoon butter in a skillet over medium heat. Add the bread crumbs and cook while stirring occasionally for 2 minutes until golden. Transfer to a large bowl.
Drain the raisins and add to the bread crumbs along with the apple slices, walnuts (if using), 1/4 cup sugar, lemon juice, cinnamon, and salt. Mix thoroughly.
Lay a sheet of parchment paper (50×40 cm) on your work surface with the longer side facing you.
Place the first filo sheet on the parchment with the short side facing you. Brush with melted butter and sprinkle with 1/2 teaspoon sugar. Position the second sheet to the right, overlapping the first sheet by 2.5 cm. Brush with butter and sprinkle with 1/2 teaspoon sugar. Continue layering the remaining filo sheets to the right with 2.5 cm overlaps, brushing each with butter and sprinkling with 1/2 teaspoon sugar.
Spread the apple filling over the layered filo dough, leaving an 8 cm border at the bottom edge and 5 cm borders on each side.
Fold the empty side borders over the filling, then fold up the bottom edge, and roll the dough tightly into a log.
Carefully transfer the strudel to the prepared baking sheet with the seam facing downward. Brush all over with the remaining butter and sprinkle with the remaining sugar.
Using a sharp knife, make shallow cuts across the top of the strudel at 2.5 cm intervals so the dough doesn't puff excessively during baking.
Bake in the preheated oven for 15 minutes until the top is golden brown. Remove from the oven and let cool on the baking sheet for 40 minutes. Dust the cooled strudel with powdered sugar and serve.
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