Remove the butter from the refrigerator 30 minutes before starting, so it can soften slightly. Also soak the raisins in lukewarm water.

In a bowl, combine the flour, baking powder, and butter cut into small pieces.
Rub the ingredients together with your fingertips until the mixture resembles breadcrumbs.
Add the sugar, egg, milk, grated lemon zest, and salt.
Knead quickly until you obtain a smooth, even dough.

Wrap the dough in plastic wrap and refrigerate for 30 minutes.

For the filling:

Cut the California prunes into small pieces and soak them in the rum.
Peel the apples and slice them thinly.

Place the apples in a bowl with the sugar. Add the prunes with their soaking liquid, pine nuts, breadcrumbs, lemon zest, and cinnamon.
Mix well to combine all the ingredients and set aside.

Roll out the shortcrust pastry on a sheet of floured baking paper, forming a rectangle approximately 40 × 26 cm.

Spread the filling over the center and press it down gently with your hands.
Using the baking paper, carefully fold the pastry over the filling, sealing the edges and ends well.

Once assembled, refrigerate the strudel for at least 30 minutes.
Brush the surface with the beaten egg and bake in a preheated oven at 180°C (350°F) for about 35 minutes, until the pastry is golden brown.

Let cool before serving. If desired, dust with powdered sugar.