Mix the flour and salt in a bowl. Whisk the egg, oil and water, add to the flour and stir with a wooden spoon. On a lightly floured surface, knead into a smooth dough. Shape into a ball, then leave to rest for approx. 30 mins. at room temperature under a bowl rinsed in hot water.
Mix the sultanas and rum, leave to steep for approx. 10 mins. Mix in the apples and all the other ingredients up to and including the cinnamon.
On a lightly floured surface, roll out the dough as thinly as possible into a rectangle and place on a tea towel. With floured hands, carefully stretch the dough from the middle outwards until it is completely translucent. Brush the dough with a little butter, sprinkle with almonds. Spread the filling over the bottom half of the dough, leaving a border of approx. 4 cm, fold the pastry ends inwards. Loosely roll up the strudel, using the tea towel to help you. Place it seam-side down on a baking tray lined with baking paper, brush with a little butter.
Approx. 35 mins. in the centre of an oven preheated to 200°C. Brush the strudel with the remaining butter several times during baking. Remove from the oven, allow to cool slightly on a cooling rack.