Prepare the dough In a bowl, mix flour and salt. Add water, oil, and vinegar, and knead until smooth. Cover and let rest for 30 minutes.
Make the filling In a bowl, combine ground walnuts, sugar, cinnamon, and milk until it forms a spreadable paste.
Roll out the dough On a lightly floured surface, roll the dough into a large rectangle or stretch gently with your hands until thin.
Add filling Spread the walnut filling evenly over the dough, leaving a small border on the edges.
Roll the strudel Carefully roll the dough from one side to the other to form a log. Pinch the edges to seal.
Bake Place the strudel on a baking sheet lined with parchment paper. Brush with melted butter. Bake at 350°F (175°C) for 30–35 minutes or until golden brown.
Cool and serve Let the strudel cool slightly before slicing. Dust with powdered sugar if desired.
