In a bowl, mix the egg with all the other ingredients up to and including the poppyseeds, allow to stand for approx. 10 mins.
Carefully unfold the pastry sheets, brush with a little butter. Stack the sheets on top of each other. Spread the poppyseed mixture over 3/4 of the dough, leaving a border of approx. 3 cm, fold the pastry ends inwards. Loosely roll up the strudel, place it seam-side down on a baking tray lined with baking paper, brush with the remaining butter.
Approx. 25 mins. in the centre of an oven preheated to 200°C. Remove from the oven, allow to cool a little on a rack and dust with icing sugar.