For the dough, put the flour, salt and 3 tablespoons of oil in a bowl. Gradually stir in 125 ml of lukewarm water. Then knead the dough until it separates from the edge of the bowl. Brush with oil and let rest, covered, for 30 minutes.
For the filling, boil the potatoes until soft, peel and press through the potato ricer while still hot.
Clean the chanterelles dry, cut larger ones in half. Peel the onion and finely chop the herbs.
Heat oil in a pan and fry onions until golden. Add the mushrooms and fry. Remove from heat. Mix with the potatoes and the yolk and season with the herbs and spices.
Roll out the dough on a floured work surface in the shape of a rectangle (or prepare the ready-made strudel sheets according to the package instructions).
Spread the mushroom filling on the bottom third. Roll up into a strudel and place seam side down on a baking sheet lined with baking paper.
Brush the mushroom strudel with the egg white and bake in the preheated oven at 180 °C for about 20 minutes until golden brown.