Heat the oven to 375°F (190°C). Line a baking sheet with parchment.
Toss the sliced apples with raisins, sugar, cinnamon, lemon juice, and walnuts if using. Let stand for 10 minutes, then drain off excess liquid.
Toast the breadcrumbs in a dry skillet over medium heat until lightly golden, about 3 minutes. Cool slightly.
Lay one phyllo sheet on a clean towel and brush lightly with melted butter. Stack the remaining sheets on top, brushing each one. Keep unused phyllo covered with a damp towel.
Sprinkle the breadcrumbs in a strip along one long side of the pastry, leaving a border. Spoon the apple mixture over the crumbs. Fold the short edges inward, then use the towel to roll the pastry into a log with the seam underneath.
Transfer to the baking sheet, brush the top with remaining butter, and cut a few small vents. Bake 35 to 40 minutes, until crisp and deep golden. Cool at least 20 minutes before slicing; dust with powdered sugar.