Position rack in middle of oven. Preheat oven to 350°F/180°C. Line a rimmed half-sheet pan with parchment paper; set aside.

Stir the finely diced apples, brown sugar, raisins, walnuts, cornstarch, cinnamon and lemon zest together in a small bowl until well combined.

Lay one piece of filo on your work surface horizontally (I like to do this on a clean piece of parchment paper) and brush with butter. Add another sheet, more butter and continue until you have used 4 sheets of filo total, finishing with brushing butter on top. Sprinkle with half of the panko all over (do this by eye) then spread half of the apple filling in a “log” shape along the long end near you. Firmly roll the log up away from you, folding in the sides as you approach the last half. Place seam side down on prepared pan. Repeat with remaining filo and filling. You should have two logs, spaced evenly apart on your baking pan. Brush tops with melted butter. (You may hold the strudel at this point, covered with plastic wrap and refrigerated, up to overnight. Some folks have even had success with freezing. Make sure to bring to room temperature before baking.)

Bake for about 15 minutes or until golden brown and crispy. Allow to cool on pans set on racks for at least 5 minutes. Use a sharp serrated knife to cut crosswise into 10 pieces each. Serve warm or at room temperature. These are best served the day they are baked.