Combine flour, salt, egg and water in a large bowl. Stir vigorously until mixture forms a dough. Turn out onto a floured surface and knead until smooth and elastic, about 10 minutes. Cover and rest for 60 minutes.
Preheat the oven to 200 C / Gas 6. Grease a 22x33cm baking tin.
Grind 1/2 the poppy seeds in a food processor. Combine ground poppy seeds, remaining whole poppy seeds, sugar, cinnamon and nutmeg in a bowl. Mix well and set aside.
Place dough on a large work surface covered with a cotton cloth. Stretch dough slightly to form a large rectangle. Pour melted butter over dough to moisten. Carefully stretch dough into the largest rectangle your work surface will allow. The dough should be thin and translucent.
Sprinkle poppy seed mixture evenly over stretched dough. Toss apples with 1/2 the double cream and all the vanilla, then spread evenly over the poppy seed mixture. Roll each side of the dough carefully to the middle and fold ends under the strudel. Cut the strudel in half and place two halves side by side on the prepared tin. Sprinkle with remaining cream.
Bake in the preheated oven until golden and apples are tender, about 60 minutes.