For the grape and curd strudel, first prepare the strudel dough. Sift the flour onto the work surface and add the egg, butter, a pinch of salt, oil and a little water into a small well. Quickly knead into a smooth strudel dough and let it rest at room temperature for about 1 hour.
In the meantime, prepare the curd filling. Beat the egg whites with the powdered sugar until stiff.
Slit the vanilla pod lengthwise and scrape out the pulp. Mix the vanilla pulp, granulated sugar, butter, a pinch of salt, vanilla custard powder, egg yolks and a dash of lemon juice until well blended. Stir in the curd and crème fraîche or sour cream, then gently fold in the beaten egg whites.
Melt the butter in a small saucepan.
Flour the work surface and roll out the dough thinly until you can stretch it out thinly in all directions with your hands (called “strudel pulling”).
Place the strudel dough on the floured work surface and brush with the melted butter.
Wash the grapes and cut them in half. Place the curd filling on the bottom third and fold in the grapes. Fold in the ends and roll up the strudel well to prevent the filling from spilling out during baking.
Line a baking sheet with parchment paper and carefully place the curd strudel on it, seam side down.
Beat the egg and mix it with some melted butter. Brush the grape and curd strudel with it and bake in the preheated oven.