Combine flour and salt in a bowl. Add water, oil, and vinegar and knead well with the dough hook of a hand mixer/ food processor or simply with your hands.
As soon as the dough appears relatively smooth, form a ball and transfer into an oiled bowl. Turn the dough over a few times to coat it lightly with oil (this will make the dough smooth and the stretching easier). Cover with plastic wrap and refrigerate for 1 hour.
In a bowl, combine raisins with rum, lemon juice, and peel and marinate for at least 30 minutes.
Core and quarter apples, then thinly slice quarters again. Grind walnuts and add to marinated raisins, together with apples, shaved almonds, and cinnamon. Mix everything well and sweeten or season to taste.
Line a tabletop with a tablecloth/ tea towel and sprinkle with flour. Take dough out of the fridge and roll with a rolling pin on cloth, trying to keep it rectangular. Then put your hands under the dough and use the backs of your hands (not your fingertips and nails!) to gently stretch it out.
Once the dough is no longer easy to manage with your hands or seems to tear, lay it down on the covered table and use your fingers to pull gently around the edges to make sure they aren't to thick. Cut any thick excess dough, if necessary.
Melt margarine and preheat the oven to 350° F (180° C) (fan oven).
Spread margarine on dough. Lay out apple filling onto the bottom portion of the dough in row that's about two inches shy of either side.
Fold over the edges left and right first, then carefully roll the strudel by using the fabric. Optionally spread every layer of the dough with margarine before further rolling.
Lift up the rolled strudel (with the fabric) and place it, seam side down, on a baking sheet lined with parchment paper. If it's too long to fit, form it into a horseshoe shape instead. Spread with remaining margarine and bake for about 40-45 minutes.
Add milk and cream to a pot and bring to a boil.
Meanwhile whisk together starch with vanilla pulp, sugar and 6 tbsp of the (still cold) milk oat mixture.
As soon as the milk starts boiling, remove from the stove and stir in starch mixture. Place back on the stove and keep whisking until the sauce has thickened to your desired consistency.
Dust apple strudel with confectioner's sugar (optional) and cut to portions.
Serve with warm vanilla sauce or vanilla (n) ice cream.