Make the dough by pulsing the gluten free flour, tapioca starch, xanthan gum, and salt in a food processor. Add the cold butter and pulse until pea-sized pieces remain. Add the sour cream, egg, and apple cider vinegar, then pulse until the dough comes together.
Wrap in plastic and refrigerate for 1 hour. Toss the sliced apples with the granulated sugar, cinnamon, nutmeg, and raisins in a large bowl. Let the mixture sit for 10 minutes to release some juices, then drain off any excess liquid.
Toast the gluten free breadcrumbs in the 2 tablespoons of melted butter in a small skillet over medium heat until golden and crispy, about 3 minutes. Set aside to cool.
Preheat your oven to 375 F and line a baking sheet with parchment paper. Roll the dough out on a well-floured piece of parchment into a large rectangle, roughly 12 by 16 inches. The dough should be thin but not transparent.
Sprinkle the toasted breadcrumbs in a strip along the center third of the dough. This layer absorbs moisture and keeps the strudel crispy. Pile the apple filling on top of the breadcrumbs and scatter the chopped walnuts over the apples.
Fold both sides of the dough over the filling, overlapping in the center. Pinch the seam and tuck the ends under. Carefully transfer to the prepared baking sheet, seam side down. Brush generously with the beaten egg wash.
Bake for 35 to 40 minutes until the strudel is deep golden brown and crisp. Let it cool for 15 minutes, then dust generously with powdered sugar. Slice and serve warm.