Preheat the oven to 375°F (190°C), and prepare a large baking sheet by lining it with parchment paper to prevent sticking.

In a spacious mixing bowl, combine the apple slices with both the white and brown sugars. Add the cinnamon, nutmeg, lemon juice, raisins (if using), and chopped nuts. Stir thoroughly to ensure the mixture is evenly coated and set aside to allow flavors to meld.

On a clean kitchen towel, lay out one sheet of phyllo pastry. Using a pastry brush, gently coat it with melted butter, then scatter a small amount of toasted breadcrumbs over the surface. Continue layering in this manner for 6 to 8 sheets, always brushing each layer with butter and dusting lightly with breadcrumbs. These crumbs help prevent sogginess between layers.

Place the prepared apple filling along one long side of the stacked phyllo, keeping a 2-inch border from the edge to avoid overfilling.

Using the towel as a guide, carefully roll the dough over the filling, forming a log shape similar to a jelly roll. Tuck the ends inward to seal the filling, and gently transfer the log to the parchment-lined tray, positioning it seam-side down.

Brush the exterior generously with additional melted butter. Transfer to the oven and bake for 35 to 40 minutes, or until the pastry turns crisp and a deep golden hue develops.

Once baked, allow the strudel to rest for 10 minutes. Finish by dusting the top with powdered sugar. Slice into portions and serve either warm or cooled to room temperature.