Combine the raisins and dark rum in a small bowl. Set aside to plump for at least 20 minutes.

In a mixing bowl, combine the flour, egg, lukewarm water, sunflower oil, and salt. Knead 8-10 minutes until smooth and elastic. Shape into a ball, wrap, and rest at room temperature for 30 minutes.

Melt the 50 g butter in a 20 cm / 8-inch skillet over medium heat. Add the breadcrumbs and toast, stirring often, for 3-4 minutes until deep golden. Set aside to cool.

Toss the sliced apples with sugar, cinnamon, lemon zest, and lemon juice in a large bowl. Let sit while you stretch the dough.

Heat the oven to 180 C / 355 F and line the sheet pan with parchment.

Lay a large clean kitchen towel on your counter and dust with flour. Roll the dough out as thin as you can with a rolling pin, then use the backs of your hands to stretch it gently from underneath until it's paper-thin and roughly 60 x 40 cm. Patch any small tears with a scrap of dough.

Brush the stretched dough with a little melted butter. Scatter the toasted breadcrumbs over the bottom two-thirds of the dough.

Drain the raisins and scatter them, along with the walnuts, over the breadcrumbs. Spoon the apple mixture on top, leaving a 3 cm border on all sides.

Using the towel to help lift and guide, roll the strudel tightly from the filled end, tucking the sides in as you go. Transfer seam-side down to the lined sheet pan.

Brush the top of the strudel with the remaining melted butter. Bake for 35-40 minutes until the crust is deep golden brown and the apples feel tender when pierced through the seam with a knife.

Cool on the pan for 15 minutes so the filling sets. Dust with powdered sugar, slice, and serve warm.