Preheat oven to 350°F (180°C).
Take out the phyllo pastry from the fridge and let it rest for 20 minutes at room temperature.
Peel the apples, remove their seeds, and dice them into half-inch cubes. Chop half of the walnuts and grind the rest.
Add the diced apples, sugar, cinnamon, raisins, lemon juice, and 30 ml. of water in a pot. Cover and let cook for about 5 minutes. Uncover, increase heat to high, and cook until thick, about 2-3 minutes. Transfer the apples into a cup and let them cool off.
Melt 2 tbsp of the butter on the stove and add the breadcrumbs. Roast the breadcrumbs until golden and set them aside.
Melt the rest of the butter and set it aside.
Mix the breadcrumbs into the apples once the apple mixture cools off. Add the crushed and ground walnuts to the mixture.
Take the pastry sheets out of their package, unroll them, and cover them with a damp towel.
Brush the pastry sheets with the melted butter and layer them on top of each other.
Place the apple filling at one end of the pastry sheets, about 2 inches from the edge, and tuck in both sides. Brush the sides with the melted butter and roll the strudel into a log.
Line a baking pan with parchment paper, brush it with some of the butter, place the apple strudel, and brush it with butter.
Bake for half an hour or until golden brown.
Transfer onto a cutting board, let cool down for 3 minutes, and cut into six pieces.
Let the apple strudel cool off for another 20 minutes and sprinkle it with powdered sugar.