In a large bowl, combine the flour and salt.
Make a well in the center and add the oil, egg, and warm water.
Mix until the dough comes together. If the dough is too sticky, add a bit more flour; if too dry, add a little more water.
Knead the dough on a floured surface for about 5-7 minutes until smooth and elastic.
Cover the dough with a clean cloth and let it rest for 30 minutes.
If using fruit, peel, core, and slice it if necessary. Mix with sugar and cinnamon if desired.
If using poppy seeds, mix with sugar and a bit of water or milk to form a thick paste.
After the dough has rested, divide it into two portions.
On a floured surface, roll out each portion into a thin rectangle, about 1/8 inch thick. The dough should be very thin and almost translucent.
Spread a thin layer of breadcrumbs over the dough if using. This helps absorb any excess moisture from the filling.
Spread the fruit or poppy seed filling evenly over the dough, leaving a small border around the edges.
Carefully roll up the dough into a log or jelly roll shape, tucking in the edges as you go to seal the filling.
Preheat the oven to 375°F (190°C).
Place the rolled Rétes on a baking sheet lined with parchment paper.
Brush the top with beaten egg for a golden finish.
Bake for 30-35 minutes, or until the dough is golden brown and crisp.
Let the Rétes cool slightly before dusting with powdered sugar.
Slice into pieces and serve warm or at room temperature.
Enjoy your meal!