Cream the butter, add the sugar and continue to stir until the mixture becomes light and creamy. Gradually add the yolks and the lemon zest while continuing to stir. First fold in the sour cream or crème fraiche, then the curd.
Whip the egg whites with salt and sugar until not too stiff, then fold into the curd mixture with a whisk.
Brush the strudel dough with melted butter and sprinkle with breadcrumbs. Spread the curd mixture on top, leaving the edges free. Roll up the dough with a cloth, wrapping the edge. Place on a greased baking tray and brush with melted butter.
Place the strudel in the preheated oven at 180 °C convection oven (210 °C top/bottom heat) and bake for about 45 minutes until golden brown.
Cool slightly, dust with powdered sugar, cut into pieces and serve with vanilla sauce.