1. Knead all dough ingredients for about eight minutes, until you have a smooth dough.
2. Shape into a ball and wrap with plastic wrap. Let rest in the fridge for at least an hour.
3. Spread a clean sheet on your work surface and sprinkle with flour. Using your hands, carefully spread the dough until it is as thin as possible, without tearing it. Brush melted butter over the dough and sprinkle with the ground walnuts. This will create a division between each layer.
4. Combine all ingredients for the apple filling. Place a row of filling at one edge of the dough. Remove the thick outer edges of the dough. Roll the dough together with the filling, using the sheet to help you.
5. Divide in the middle into two parts lengthwise, about 12 inches (30 centimeters) long, and close the ends.
6. Place strudel on a baking pan lined with parchment paper. Brush with melted butter and sprinkle the rest of the ground walnuts on top.
7. Bake at 390 degrees Fahrenheit (200 degrees Celsius) for 22–25 minutes, until golden.
8. Remove from oven and brush with melted butter. Cool for 10 minutes and serve.