To prepare the dough: Whisk the egg, butter, and salt with about 3/4 cup cold water. Combine just enough of the liquid with the flour to make a soft dough. The dough should feel resilient but remain very slightly sticky. Drizzle in a tablespoon or more water if the dough is too ragged, or sprinkle in a small amount of flour if it is too moist and mushy. Knead for five minutes by hand, (2 minutes if using an electric mixer) until smooth, soft and warm. Form it into a smooth ball, wrap in plastic film, and let rest for at least 1 hour, preferably 4 hours.

To prepare the filling: Toss together in a medium sized bowl, the apple slices and lemon juice. Add the walnuts, raisins, sugar, and cinnamon and toss until evenly coated.

To assemble: Spread a table with a clean cloth and sprinkle evenly with flour. Place the dough in the center of the table, flatten with your palms into a 1-inch thick round, and then begin stretching the dough from the center to the edge. Use a combination of the sides of your hands and the tops of your knuckles to work the dough gently and evenly. Continue stretching the dough until you can see light through it. If there are occasional holes at the center, just pinch together to seal. Don't worry about holes or tears near the edge of the dough.

Brush the entire strudel dough evenly with butter. Sprinkle half the crumbs on one half of the dough, and on the same side arrange the apple filling in a long row close to the edge of the dough. Sprinkle with the remaining crumbs. Fold over the edge and roll the strudel into a log shape, using the cloth to help support and roll the pastry. Tuck in the sides and roll up completely, arranging the strudel seam side down.

Transfer carefully to a buttered or parchment-lined baking sheet, curve the log into a crescent shape, if needed, to fit the roll onto the pan. Brush with butter and chill for at least 30 minutes (or overnight).

Preheat the oven to 350°F. Bake the strudel until golden brown, 30 to 40 minutes. Let cool slightly before cutting and serving.