Combine butter, milk, the dried yeast, sugar and egg yolk in a medium bowl and mix together with a whisk.
Add the flour, salt and lemon zest and use your hands to work the ingredients into a pliable dough. The dough should not be stiff but also not too sticky. It should be nice and soft and easy to shape. Knead for 5 minutes.
Place a cover on the bowl with the dough. Keep at room temperature/in a wam place until the volume of the dough has expanded sufficiently.
Prepare the filling about 15 mins before your dough is ready to be shaped.
To do this, add the water, butter, honey (or sugar), Powidl, ground cloves (or cinnamon) into a pan, heat up until it starts bubbling up.
Take the pan off the heat and add the ground porridge oats, poppy seeds, currants and rum and combine well.
Now back to the dough which should have almost doubled in volume by now. On a slightly floured surface, roll out the dough into a 30 x 20 cm rectangle. Use your dough scraper to chop some of the protruding edges off in order to get to the rectangular shape.
Distribute the filling evenly across the dough rectangle, leaving 0.5 cm around the edges free.
Roll up the dough lengthwise. Make sure that no air bubbles get trapped as you do this. Try not to add more length to the Strudel as you roll it up, it should still end up being 30 cm long.
Place the Strudel seam side down onto a baking tray lined with baking paper. Flatten it slightly in order to achieve a more oval than round shape without lengthening it.
Cover with a kitchen towel and leave to rest for approx. 1 hour (depending on the temperature in your room).
Preheat the oven to 180°C.
Just before the bake, brush the Strudel with egg wash.
Bake for 30 minutes or so on the second lowest shelf of your oven, until golden brown.
Cool slightly before serving.