Make the dough. In a deep bowl, beat the egg with salt and vegetable oil. Dilute the mixture with warm water and add baking powder. While stirring, pour flour into the bowl until a thick, homogeneous mass is formed. Leave 2-3 tablespoons of flour for kneading and rolling out the dough. Put the mass on the table and knead a dense and elastic dough. Roll the dough into a ball, cover it with cling film and leave in a warm place for 20 minutes.
Prepare the filling. Drain the water in which the raisins were steamed and dry them on a towel. Cut the apples into cubes about 1 cm in size. Mix together the apples, raisins, walnuts, add sugar and cinnamon.
Form the strudel. Sprinkle a little flour in a thin, even layer on the table and put the dough on it. Form the dough into a flat rectangle and gradually stretch it, moving it from the middle to the corners and carefully pulling out the sides. The result should be a thin rectangular plate. Melt the butter. Lightly grease the surface of the dough with butter using a culinary brush. Spread the filling on the dough in a thin, even layer, leaving at least 2 cm from the edge. Roll the roll, trying not to make it tight, carefully seal the edges.
Bake the strudel. Preheat the oven to 180 degrees. Spread baking paper on a baking sheet and lay out the strudel. Grease it with butter and put it in the oven. Bake for about 40 minutes until a stable golden brown crust appears. Grease the finished apple strudel with butter again, let it cool a little and cut into portions.